Preheat oven to 350°F and grease a 9×9" pan. Lightly dust with flour or line with parchment paper or foil. Gather ingredients.
Whisk the butter and sugar in a large bowl or use a handheld mixer. Add in egg and vanilla and stir until combined. Then add in the peanut butter and mix until combined.
Mix in the flour, baking powder and salt. Fold in Reese's Pieces and chips and mix just until combined.
Press the dough into the pan (flour your hands if you need to). Press extra Reese's Pieces and peanut butter chips on top if you’d like.
Bake for 18-23 minutes or until a toothpick comes clean. The dough should be soft in the center and the edges lightly golden.
Allow bars to cool and then slice into 9 bars.