Adjust the oven rack one-third up from the bottom and preheat the oven to 375 degrees. Prepare a 9-inch-square pan as follows:
Turn the pan upside-down. Cut a 12-inch square of aluminum foil, center it over the inverted pan and fold sides and corners down. Remove the foil, turn the pan upright and place the foil in the pan. In order not to tear the foil, use a folded towel or pot-holder to press it firmly into place in the pan. Brush with [oil (tester's note)] or very soft or melted butter. Set prepared pan aside.
[Tester's note: If desired, toast the nuts in a large frying pan over medium-low heat, stirring frequently, until they begin to color and become fragrant, about 7 minutes. Immediately pour them out of the pan and set aside to cool.]
Place the chocolate in the top of a small double boiler over hot water on moderate heat. Cover until partially melted, then uncover and stir until completely melted. Remove the top of the double boiler and set it aside, uncovered, to cool the chocolate slightly.
In the large bowl of an electric mixer, cream the butter. Add the salt, coffee, pepper, and vanilla; blend briefly. Then add the sugar and beat until blended.
Add the eggs one at a time, scraping bowl with a rubber spatula and beating only until incorporated after each addition. On low speed add the chocolate and then the flour, scraping the bowl with a rubber spatula and again beating only until incorporated.