Preheat oven to 350 degrees. Shell and roughly chop hazelnuts. Heat a large frying pan over low-medium heat. Arrange hazelnuts in a single layer in pan. Stir until hazelnuts start to turn golden brown. Remove from heat.
Using an electric mixer, cream together butter and sugar until light and fluffy. Add eggs and coffee liqueur. Mix well.
With the mixer on low speed, slowly add in flour, cocoa, and baking powder. Stir in toasted hazelnuts.
Line a cookie sheet with parchment paper. Divide the dough into two halves. Shape the dough in two loaves about 10 inches long. Place loaves on parchment paper about 4 inches apart (loaves will expand while baking).
Bake for 20 to 25 minutes. Tops of loaves should be firm to the touch. Remove the loaves from the oven and carefully place on wire racks. Cool for about 10 minutes.
Use a serrated knife and slice each log into 1/2-inch pieces. Return biscotti to the baking sheet and bake for 20 more minutes, or until cookies are crisp and toasty. Allow biscotti to cool completely. Store cookies in airtight container.