Preheat oven to 375 degrees F. Grease a large cookie sheet; set aside. In a large bowl, beat butter with an electric mixer on medium to high speed for 30 seconds. Add sugar, cocoa powder, and baking powder. Beat until combined, scraping side of bowl occasionally. Beat in eggs until combined. Beat in as much of the flour as you can with the mixer. Using a wooden spoon, stir in any remaining flour. Stir in white baking pieces and semisweet chocolate pieces.
Divide dough in half. Shape each half into a 9-inch-long roll. Place rolls on prepared cookie sheet; flatten each to about 2 inches wide.
Bake for 20 to 25 minutes or until a toothpick inserted near center comes out clean. Cool on cookie sheet for 1 hour. Reduce oven temperature to 325 degrees F.
Transfer rolls to a cutting board. Using a serrated knife, cut each roll diagonally into 1/2-inch-thick slices. Place slices, cut sides down, on an ungreased cookie sheet. Bake in the 325 degree F oven for 8 minutes. Turn slices over and bake for 7 to 9 minutes more or until dry and crisp (do not overbake). Transfer cookies to a wire rack and cool. Makes 32 cookies.