Preheat the oven to 350 degrees F (175 degrees C). Lightly oil a baking sheet.
Combine sugar and butter in a large bowl and mix until light and fluffy. Add eggs 1 at a time and blend well.
Mix flour, coffee, cocoa powder, and baking powder in a bowl and whisk to blend. Slowly add flour mixture to butter mixture and stir until dough is blended.
Lightly oil a large square of aluminum foil. Place dough on top and use foil to roll into a log about 12 inches long. Open the foil and slip dough log onto the prepared baking sheet. Gently flatten log to about 1 1/2 inches in height.
Bake in the preheated oven for 25 minutes. Remove and let cool 10 minutes, leaving oven on. Slice log into 12 biscotti using a serrated knife. Stand biscotti upright on the baking sheet, leaving 1/2 inch space between each.
Return to the oven and bake 12 minutes more. Remove and let cool completely, about 1 hour. Store biscotti in an airtight container.