Made with both cocoa powder and dark chocolate, these cookies have a bold flavor that goes well with espresso. This recipe is one of our Better Basics: 10 New Takes on Family Favorites, see the others.
Preheat oven to 350 degrees. Whisk together flour, cocoa powder, baking powder, and salt in a bowl.
Melt chocolate and butter in a heatproof bowl set over a pan of simmering water, stirring until smooth and combined. Remove from heat and whisk in brown sugar, then eggs and vanilla. Stir in flour mixture with a flexible spatula, then fold in 1 cup walnuts. Refrigerate until dough is just firm, about 30 minutes.
Drop heaping tablespoons of dough, at least 2 inches apart, onto 2 parchment-lined baking sheets. Sprinkle with remaining 1/4 cup walnuts. Bake until tops of cookies are set and shiny, about 12 minutes. Let cool completely on sheets on wire racks.