In a large mixing bowl, beat butter with an electric mixer on medium speed for about 30 seconds. Add sugar, cocoa powder, and baking powder, and beat until combined. Next, beat in eggs.
Beat in as much of the flour as you can. Using a spoon, stir in any remaining flour, white baking pieces, and semisweet chocolate. Divide dough in half.
Shape each half into a 9-inch-long log, and place these logs, about four inches apart, on a greased cookie sheet. Flatten them slightly until about two inches wide.
Bake logs for 20 to 25 minutes, or until a wooden toothpick inserted near the centers comes out clean.
Cool on the cookie sheet on a wire rack for 1 hour (you can also wrap the logs in plastic and let stand overnight).
After you cool the logs, slice them diagonally into biscotti-sized pieces. Place them cut side down on a fresh parchment-lined cookie sheet. Bake for 7 to 9 minutes on each side, or for about 12 minutes total. Just watch them for the correct doneness to avoid burning.