1. Lightly grease large cookie sheets; set aside.
2. In a heavy saucepan, heat one package of the semisweet chocolate pieces, the unsweetened chocolate and the butter until melted, stirring constantly. Transfer to a large mixing bowl to cool slightly.
3. Mix flour, baking powder and salt. Set aside.
4. Add eggs, sugar, and vanilla to chocolate mixture; beat with an electric mixer. Add flour mixture and beat on low speed till well mixed. Stir in remaining chocolate pieces and nuts.
5. Using 1/4 cup of the dough for each cookie, drop the dough 3 inches apart on lightly greased cookie sheets. Flatten slightly.
6. Bake the cookies in a 350 degree F oven for 12 to 15 minutes or until the edges are firm and the surface is dull and cracked. Let stand for 2 minutes on cookie sheets. Remove; cool on wire racks. Drizzle with melted white chocolate and semisweet chocolate. Makes 24 cookies.