Preheat oven to 350°F. Lightly grease an 8- inch square baking pan.
Blend baking mix, baking powder, and salt, set aside. Microwave chocolate and butter in glass measuring cup on high for 1 minute, stir until chocolate is completely melted. In a large bowl mix sugar, eggs and vanilla together then stir in melted chocolate. Add dry ingredients; blending well. Fold in chopped nuts and spread batter in prepared pan. Bake 30-35 minutes. Or until toothpick inserted in center comes out clean. Cool on wire rack. Cut into squares and serve.