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Preheat oven to 350°F. Line a baking sheet with parchment paper and set aside.
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In the bowl of a stand mixer, beat sugar, eggs, melted butter, instant coffee and vanilla extract on medium-low speed until well blended. Add in flour, cocoa powder, and baking powder, mixing on low speed until almost completely mixed. Scrape down sides of bowl, add desired mix-ins, and mix until completely incorporated.
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Turn out dough onto prepared baking sheet and form into a long, 2 inches thick and about 12 inches wide. Bake for 25 minutes.
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Remove from oven and allow to sit until cool enough to handle. Using a serrated knife, cut cookies into pieces about 3/4-inch thick, on the diagonal. Turn cookies cut-side down and bake for another 12-15 minutes. Then, remove from oven and flip over. Cook another 15 minutes, until cookies are toasted on both sides.
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Remove from oven and allow cookies to cool on a wire baking rack.
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Store in an airtight container at room temperature.