Preheat oven to 325F.
Lightly grease an 8-by-8- by-2-inch baking pan.
Sift flour with baking powder and salt.
In small bowl of electric mixer, at high speed, beat butter with sugar until light and fluffy; beat in eggs, one at a time, until very light.
Beat in melted chocolate and vanilla.
At low speed, blend in flour mixture just until combined.
Stir in nuts.
Turn into prepared pan, spreading evenly.
Bake 30 minutes.
Cool 10 minutes.
With sharp knife, cut into squares.
Let cool completely in pan.