Melt chocolate in 1-quart batter bowl, using MEDIUM heat for 2-3 minutes.
Add butter and sugar.
Microwave on 50% power 1-2 minutes, or until mixture is smooth.
Add eggs and vanilla, and beat until smooth.
Stir together flour, baking powder and salt.
Stir into chocolate mixture and add nuts.
Spread into an 8 x 8 x 2" greased baking dish.
Microwave 5 minutes on HIGH.
Cool and spread on Mocha Frosting.
A delicious treat!