Melt 1 cup of the semisweet chocolate chips, the unsweetened chocolate, and butter in a heavy medium saucepan, stirring constantly. Transfer to a large mixing bowl; cool slightly.
Meanwhile, stir together the flour, baking powder, and salt; set aside. Add eggs, sugar, and vanilla to chocolate mixture. Using a wooden spoon, beat until combined. Stir in the flour mixture, remaining chocolate chips, and nuts.
Drop dough by heaping teaspoonfuls onto the prepared baking sheet.
Bake at 350 degrees for 8-10 minutes, or until edges are firm and surface is dull and cracked. Cool on cookie sheet for 1 minute. Transfer to a wire rack; cool completely.
Drizzle each cookie with melted semisweet chocolate.