Preheat oven to 350 degrees F.
Butter and flour an 8-inch square pan.
In a large saucepan, slowly melt the butter, careful not to let it bubble. Remove from heat, and stir in sugar, eggs, and vanilla until blended.
Add cocoa, flour, salt, and baking powder, and blend until smooth.
Pour into cake pan. Bake for 25 to 30 minutes.