In a medium bowl using an electric mixer on medium speed, beat together the butter, sugar and peanut butter until smooth and no butter or sugar lumps remain. Add in the egg and vanilla and continue beating until combined (about 30 seconds).

Turn the mixer down to low speed and carefully beat in the flour, cornstarch and baking powder. You may need to turn off the mixer and scrape down the sides of the bowl.

Spoon the batter into your prepared pan, and press down to the edges using your hands or a rubber spatula. Bake in your preheated oven for 20-23 minutes or until the top is set and the edges are just starting to brown and pull away from the sides of the pan. Allow to cool fully before frosting.

To make the frosting, in a medium bowl using an electric mixer on medium speed beat the butter until softened. Turn the mixer down to low speed and beat in the confectioner's sugar about ½ at a time. After each addition has been incorporated, turn the mixer up to medium high speed for 5 seconds. Then with the mixer on low speed, beat in the cocoa, vanilla and salt. Finally, beat in the whipping cream 1 tablespoon at a time until you get your desired consistency and sweetness level.

To decorate the bars, first lift them out of the pan using the aluminium foil. Frost with the chocolate buttercream using a flat-edge knife. Then decorate with Reese's Piece. Cut into 9-12 bars and serve.