Preheat the oven to 325 degrees F. Butter the sides of an 8 by 8-inch square cake pan and line the base with parchment or greaseproof paper.
Sift the flour and baking powder into a small bowl, and set aside. In a large bowl, cream the 7 tablespoons butter and peanut butter together until very soft. Add the sugar, egg, and vanilla extract, and beat until combined. Add the flour, baking powder, and the chopped chocolate, and mix to form a dough. Spread the dough in the prepared pan, and bake in the oven for 25 to 30 minutes, or until golden brown and almost firm in the center. Allow the brownies to cool in the pan, before removing and cutting into squares.