Preheat the oven to 325F, grease a 9x9 baking pan.
Melt the butter and chocolate together over low heat.
Transfer to mixer and blend with peanut butter and sugars on med-low speed until smooth.
Beat egg with vanilla and add to mixture.
Combine flour, baking powder and salt; stir into the sugar mixture.
Spread evenly into the prepared pan.
Bake for 35 minutes in preheated oven, or until firm.
TOPPING: blend together the butter, peanut butter, vanilla, and sugar. Gently mix in the whipped topping by hand. Refrigerate until brownies have cooked and cooled.