I've been making cookies all my life. I'm not exaggerating when I calculate that I've dropped tens of thousands of them onto my battered cookie sheets over the decades.

And each batch contains the basics — flour, sugar, butter, eggs — then the flavorings, spices and toss-ins that give each variety its taste: peanut butter, chocolate chips, oatmeal and raisins, molasses, ginger, cinnamon.

So imagine my surprise when I found a secret ingredient I'd never heard of that produces a cookie so tender and so tantalizing that it truly melts on the tongue.

Even more surprising is how sharp and pungent this ingredient is. Just a sniff from the jar practically brings tears to the eyes, and when the oven door is opened at the end of the baking cycle, the odor of ammonia floods the kitchen. But mysteriously, the scent vanishes once the cookies are cooled and ready to eat.

The secret ingredient is baker's ammonia, also known as bicarbonate of ammonia or by its scientific name, ammonium carbonate. And baking with it is a lesson that is equal parts chemistry and history.

While it was new to me, ammonium carbonate dates to the Middle Ages. What the Germans called hartshorn or hirschhornsalz was made from the residue or "salts" of burned and shredded horns, hooves and antlers and prized as a leavening agent.

The synthetic version, a white powder, became popular before baking soda and baking powder became widely available.

Baker's ammonia is the ingredient that makes German springerle and gingerbread cookies light and springy but also crispy. It developed a following among Greek and Scandinavian bakers, creating the melt-in-the-mouth texture of Swedish drömmar or "dream biscuits" and the Icelandic Christmas delicacy Loftkökur, or air cookies.

Today it's available at some supermarkets and specialty stores; I got mine on Amazon, paying $6.75 for a 2.7-ounce jar.

'Keeping hubby happy'

I first became aware of the wondrous properties associated with baker's ammonia when I sampled a crackled-topped cookie produced by my friend Pete Petersen, based on a cherished recipe passed down from his maternal grandmother, Inger Baadsgaard.

"I don't know how much of the tale is true and how much is family legend," admitted Petersen, who splits his time between Jacksonville, Fla., and Balsam Lake, Wis.

"The story goes that after my grandparents' wedding in the 1920s, the preacher's wife ran up while they were on the train to go on their honeymoon and shoved ahandwritten recipe through the window. She said she'd never shared it before," Petersen said. "Kind of like, 'Hey, bride, this will keep your hubby happy.' "

Petersen suspects those cookies originally had a Danish or German name, but he and his brothers called them Doughys, and gorging on them in their grandparents' south Minneapolis kitchen is a memory that remains as sharp as the scent of the secret ingredient.

"The family joke is to hand the jar to whoever you're baking with and say, 'Check this to see if it's fresh,' " he said. "They don't have to worry about plucking their nose hairs for a while."

Five cents' worth

Sleuthing the back story of baker's ammonia, I put Rae Katherine Eighmey on the case.

A food historian and cookbook author, Eighmey studied a digital bookshelf full of old community cookbooks, using "baker's ammonia" as keywords, in search of vintage recipes.

"These cookbooks were church fundraisers and provide a window into what was going on in a community. They have the name of the contributor next to the recipe," she explained. "A woman would submit only her best recipe for everyone in town to see. Her reputation was on the line; there are no clinkers in these cookbooks."

Reviewing cookbooks published between 1849 and 1921, Eighmey found one recipe with baker's ammonia that popped up repeatedly. It's for lemon crackers.

"It's not a cookie. It's barely sweet, with a mild lemon flavor that strengthens as they rest after baking," she said.

Before the turn of the last century, there were no standardized measurements like cups and teaspoons. Many of the lemon cracker recipes that Eighmey found called for "five cents of baker's ammonium." In an era before containers with tight lids were widely available, home cooks were advised to procure the pungent powder at "the druggist's," where it was doled out in paper packets.

"Baking is all chemistry, so you have to figure out the ratios," she said. "I'm used to deciphering old recipes that call for 'flour enough to make stiff,' but the conundrum with baker's ammonia was the specification for 'five cents' worth.' "

An inveterate recipe tweaker, Eighmey built a spreadsheet with 10 versions of the cracker recipe, then rolled up her sleeves and rolled out six test batches.

She kept her kitchen windows open and her vent fan on when she opened her oven door to take out the hot crackers.

"The aroma is truly sinus-clearing, but I don't know where it goes. It doesn't linger in the oven and there's none of it in the finished product," she said. "The texture of the cracker is astounding. Tender isn't the right word; it's subtle, it's flaky. I struggle to describe it. It has teeny tiny pockets of crispiness."

Eighmey notes that people with sensitive taste buds sometimes detect a soapiness or alkaline flavor in cookies leavened with today's pantry ingredients. There's no such aftertaste with the baker's ammonium.

And she cautions that baker's ammonia is only effective for thin cookies; a thicker dough doesn't let the ammonia evaporate and escape. One historic source she reviewed said using the product in a wet dough, like a muffin or cake, would give it the aroma of manure.

Baker's ammonia should be kept in a closed jar to prevent evaporation, and away from children. Sampling raw dough is also off limits. But even with the limitations associated with the ingredient, Eighmey thinks baking with it is worth the trouble.

"It's some kind of magic," she said. "I had a ton of fun fiddling with this and I'm keeping the cracker recipe. I just love them."

Doughys

Makes about 3 dozen.

Note: You'll want bake these in a well-ventilated kitchen. From Pete Petersen, who passed along this recipe of his grandmother's.

• 1/2 c. shortening, such as Crisco

• 1/2 c. butter, softened

• 1 1/2 c. sugar

• 2 tsp. almond extract

• 1 1/2 c. flour

• 2 tsp. baker's ammonia

Directions

Preheat oven to 325 degrees and grease baking sheet or line with parchment paper.

In a large mixing bowl, cream shortening, butter, sugar and almond extract.

In a small bowl, whisk together flour and baker's ammonia. Pour into the sugar mixture and stir until blended.

Drop by tablespoon onto a baking sheet. Bake about 13 minutes or until cookie is set and firm on top, but not brown. Remove from oven and cool cookies on the baking sheets for a few minutes before transferring to a wire rack to cool completely.

Lemon Crackers

Makes 6 dozen crackers.

Note: Rae Katherine Eighmey also tested this recipe using modern pantry ingredients, substituting 1 teaspoon baking powder, 1/2 teaspoon baking soda and 1/8 teaspoon cream of tartar for the baker's ammonium, but found the texture was not the same. Eighmey called these crackers "a perfect addition" to a cheese plate, noting that even the thinnest version stands up to a dollop of jam or schmear of cream cheese. The baking time for these crackers varies with the thickness of the cracker. The thinner the cracker, the more the light lemony flavor comes through.

• 1/4 c. butter, softened

• 1/2 c. sugar

• 1 egg white, lightly beaten

• 2 to 3 tsp. lemon extract

• 2 c. flour, divided, plus extra for rolling dough

• 1 tsp. baker's ammonia

• 1/4 c. milk, warmed

• 1/4 tsp. salt

Directions

Preheat oven to 350 degrees and lightly grease baking sheets.

In a large bowl, cream together butter and sugar until just blended. Stir in egg white and lemon extract. Stir in 1 cup of the flour.

Combine baker's ammonia with milk, stirring to dissolve. Add milk mixture to the batter and mix until combined. Stir in remaining 1 cup of flour and salt and knead the dough briefly until smooth.

Roll out dough on well-floured surface to a thickness of about 1/8 inch. Cut into 2-inch squares, or the shape of your choice. Place crackers on baking sheets (they can be close together as they do not spread while baking) and bake until light brown. Check after 8 minutes for the thinnest version, 10 to 12 minutes for thicker crackers (see Note).

Kevyn Burger is a Minneapolis-based freelance broadcaster and writer.