In a large mixing bowl, cream butter and sugar with an electric mixer until light and fluffy. Gradually beat in cocoa and baking powder. Beat for 2 minutes. Beat in the eggs one at a time. Stir in flour by hand. Mix in white chocolate and chocolate chips. Cover dough, and chill for about 10 minutes.
Preheat oven to 375 degrees F (190 degrees C). Divide dough into two parts, and roll each part into a 9 inch long log. Place logs on lightly greased cookie sheet, about 4 inches apart. Flatten slightly.
Bake for 20 to 25 minutes, or until toothpick inserted in center comes out clean. Cool on cookie sheet for 5 minutes, then carefully transfer to a wire rack to cool for one hour.
Cut each loaf into 1/2 inch wide diagonal slices. Place slices on an ungreased cookie sheet, and bake at 325 degrees F ( 165 degrees C) for 9 minutes. Turn cookies over, and bake for 7 to 9 minutes. Cool completely, then store in an airtight container.