1 Lightly grease a large cookie sheet. Set aside.
2 In a large mixing bowl beat butter or margarine with an electric mixer on medium speed for 30 seconds. Add the sugar, cocoa powder, and baking powder; beat until combined. Beat in the eggs. Beat in as much of the flour as you can. Using a spoon, stir in any remaining flour, white baking bar, and semisweet chocolate.
3 Divide dough in half. Shape each half into a 9-inch-long log. Place logs about 4 inches apart on prepared cookie sheet. Flatten logs slightly until about 2 inches wide.
4 Bake logs in a 375° oven for 20 to 25 minutes or until a wooden toothpick inserted near the centers comes out clean. Cool on the cookie sheet set on a wire rack for 1 hour. Using a serrated knife, cut each log diagonally into 1/2 inch-thick slices. Lay slices, cut sides down, on ungreased cookie sheets.
5 Bake slices in a 325° oven for 8 minutes. Turn slices over. Bake for 7 to 9 minutes more or until slices are dry and crisp. (Do not overbake.) Transfer to wire racks and let cool. Store in an airtight container at room temperature for up to 1 week