Preheat oven to 350°F and line a 9×9″ square pan with aluminum foil. Lightly grease the foil with cooking spray. In a large bowl, cream together butter and peanut butter until smooth, then beat in sugar until light and fluffy. Add in eggs one at a time, followed by the vanilla.
In a small bowl, stir or sift together cocoa, flour, salt and baking powder. Mix into peanut butter mixture until just combined. Stir in chocolate chips and scrape batter into pan, spreading into an even layer.
Bake for 26-29 minutes, until set. Edges should feel slightly firm and the center should not look wet or jiggly. Cool on a wire rack and lit brownies out with the foil when ready to slice.