Preheat oven to 325. In a 1 quart pan, melt chocolate and butter, stirring constantly. Remove from heat. In a large bowl, stir together sugar and flour. Add melted chocolate mixture, eggs, and coffee liqueur. Mix well. Stir in chocolate pieces and nuts.
Spread batter in a greased 13 x 9 inch baking pan. Bake for 35 minutes or til edges are firm. Center will be soft. Cool 30 to 60 minutes on a wire rack before cutting. Chill at least 2 hours before serving. Makes 48