• 6 c. (about 9 oz.) small pretzel twists, lightly broken up
To make the shortbread: Position a rack in the center of the oven and preheat to 350 degrees. Line a 9- by 13-inch baking pan with parchment paper, folding paper up and over the sides of the pan. In a small bowl, whisk together flour and 1/4 teaspoon salt, and set aside.
In the bowl of an electric mixer fitted with the paddle attachment, beat together 1 stick butter and 1/3 cup brown sugar at medium speed until combined, about 1 minute. Beat in the vanilla, change speed to low, and add the flour-salt mixture. Mix until crumbly, 10 to 15 seconds. Pat the dough evenly into the bottom of the parchment-lined pan, and prick dough all over with a fork. Bake 15 to 18 minutes, until just golden brown around the edges. Remove from oven and allow to cool slightly.
To make the caramel: In a large saucepan over medium heat, combine 1 1/2 sticks butter, 1 1/4 cup brown sugar, honey, maple syrup and 1/2 teaspoon salt. Stir regularly until mixture is foamy and slightly thickened, about 10 minutes. Add cream and cook, stirring occasionally, until a candy thermometer inserted into the caramel registers 240 degrees (soft ball stage), about 11 minutes. Add the crushed pretzels and quickly incorporate into the caramel.