Double Chocolate Biscotti

1/2 cup butter, softened (one stick)
2/3 cup white sugar
1/4 cup unsweetened cocoa powder
2 teaspoons baking powder
2 eggs
1 3/4 cups all-purpose flour
4 (1-ounce) squares white chocolate, chopped (white chocolate chips work as well)
3/4 cup semisweet chocolate chips

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 15 minutes.
Preheat oven to 375°. 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 25 to 30 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 30 minutes.
Cut each loaf into 1/2 inch wide diagonal slices. Place slices on an ungreased cookie sheet, and bake at 325°F for 12 minutes. Turn cookies over, and bake for 12 minutes more. Cool completely, then store in an airtight container.