Heat oven to 350 degrees. Grease 9 inch square pan.
Stir togeter butter, sugar and vanilla in bowl. Add eggs, beat well with spoon. Stir together flour, cocoa, baking powder and salt; gradually add to egg mixer, beating until well blended. Stir in nuts in desired. Spread batter evenly into prepared pan.
Bake 20 to 25 minutes or until brownies begin to pull away from sides of pan.Sprinkle with powder sugar. Cool completely in pan on wire rack. Cut into squares.