Preheat oven to 350 degrees. In bowl of electric mixer fitted with paddle, cream together butter and sugar until well-blended and fluffy. Add eggs and beat until well-incorporated, then beat in cocoa powder. Mix in flour, salt and baking powder until just combined. Gently fold in remaining ingredients. Transfer dough to clean work surface and gently mix dough by hand to ensure even distribution of ingredients. Divide into 12 equal portions and place each on sheet pan lined with parchment paper. Bake in oven 12 minutes, taking care not to overbake. Let cool on rack and store in airtight container.
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