Preheat oven to 450°F.
Grease an 8-inch square baking pan.
Melt butter with chocolate in medium saucepan.
Remove from heat.
Combine sugar, flour and baking powder in large bowl.
Add eggs and vanilla and beat well.
Pour in chocolate mixture and blend thoroughly
Stir in walnuts.
Pour into pan.
Bake until tester inserted in center comes out clean, about 15 to 20 minutes.
Cool completely in pan before cutting into squares.