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 190 degrees C. Divide dough into two parts, and roll each part into a 23cm long log. Place logs on lightly greased baking tray, about 10cm apart. Flatten slightly.
Bake for 20 to 25 minutes, or until toothpick inserted in centre comes out clean. Cool on baking tray for 5 minutes, then carefully transfer to a wire rack to cool for one hour.
Cut each loaf into 1cm inch wide diagonal slices. Place slices on an ungreased baking tray, and bake at 165 degrees C for 9 minutes. Turn biscuits over, and bake for 7 to 9 minutes. Cool completely, then store in an airtight container.