Preheat an oven to 375F.
In an electric mixer, cream together the butter, sugar, cocoa powder, and baking powder until light and fluffy. Add the eggs and the flavored liqueur; beat on high for 30 sec-onds. Add the flour and mix on medium just until incorporated. Add the chocolate and nuts and mix just until incorporated.
Divide the dough in half and shape into two logs. Set the logs about 4 inches apart on a lightly greased baking sheet. Flatten them slightly until they about 2-inches wide. Bake for about 20 minutes or until a toothpick inserted comes out clean. Remove from the oven and allow to cool for a few minutes. Reduce the oven's temperature to 325F.
Using a serrated knife, cut each log into 1/2 thick slices. Place them onto an un-greased baking sheet and bake for 8 minutes. Turn the slices over and bake for an additional 7 minutes. Remove from the oven and allow to cool.