Preheat oven to 350 degrees F. Line 2 baking sheets with silicone liners or parchment paper and set aside.

In the bowl of a stand mixer, cream together the butter, peanut butter, brown sugar & sugar with the paddle attachment until creamy, about 1 minute. Add in the egg, egg yolk, and vanilla and beat to combine. Lastly, add in the baking soda, dry pudding mix and flour and beat until a soft dough has formed. Fold in the Reese's Mini Pieces and peanut butter chips.

Drop the dough into heaping Tablespoonfuls onto the baking sheets, about 1-2" apart. Bake for approx. 8-10 minutes, rotating the pans halfway through baking time to ensure even cooking. Once the cookies come out, gently press down on them slightly with the bottom of a drinking glass. Cool on the racks completely.

To make the frosting: in the bowl of a stand mixer, beat together the butter, peanut butter and vanilla until smooth, about 1 minute. Add in the powdered sugar one cup at a time, until frosting is spreadable and smooth.

Spread a heaping Tablespoonful or so (more or less to suit your tastes) of the frosting onto the cooled cookies. Top with a heaping Tablespoonful of chopped Reese's PB Cups.