Preheat oven to 325 degrees F. Line a 9x13 inch pan with aluminum foil, making sure the foil overlaps the edges of the pan. Spray the foil with cooking spray and set aside.

In a medium bowl, stir together flour, cocoa powder, salt and baking powder. Set aside.

In a large bowl, stir together the melted butter, brown sugar and granulated sugar. When combined, stir in egg, egg yolk and vanilla extract until smooth. Stir in the dry ingredients until incorporated completely. Fold in both dark chocolate and peanut butter chips.

Spread the batter in the foil-lined pan in one even layer. Bake for 25 to 30 minutes, until the edges pull away from the sides of the pan. Cool for at least 15 minutes before slicing into bars and serving.