Preheat oven to 350 degrees. Prepare a 9x13 baking pan with cooking spray. Set aside.
Combine butter and brown sugar in a large bowl until mixed and light.
Add the peanut butter and stir to combine.
Mix in the eggs and vanilla extract and stir well.
Add the dry ingredients all together, flour, baking powder and salt and stir them into the batter.
Pour in the Reese's pieces and carefully mix them into the batter.
Pour the batter into your prepared pan and push around to all edges and spread evenly with a rubber spatula.
Bake for about 30 minutes or until an inserted toothpick comes out clean.
Allow to cool slightly before cutting and serving.