Preheat oven to 350 degrees F (175 degrees C). Grease and flour an 8 inch square pan.
In a large saucepan, melt 1/2 cup butter. Remove from heat, and stir in sugar, eggs, and 1 teaspoon vanilla. Beat in 1/3 cup cocoa, 1/2 cup flour, salt, and baking powder. Spread batter into prepared pan.
Bake in preheated oven for 25 to 30 minutes. Do not overcook.