Preheat oven to 350°F (177°C). Line a large baking sheet with parchment paper or a silicone baking mat.

Using a handheld or stand mixer fitted with a paddle or whisk attachment, beat the butter, granulated sugar, and brown sugar together in a large bowl on medium-high speed until combined and creamy. Add beaten egg, vanilla extract, and peanut butter and then beat on medium-high speed until combined. Scrape down the sides and up the bottom of the bowl as needed. Beat in the flour, baking soda, and salt on low speed, then beat in the Reese’s Pieces and chocolate chips until combined.

With a large spoon, scoop into a ball onto prepared baking sheet. The dough will be sticky. Make sure the cookie dough ball is tall and round. I topped the cookie dough with a few extra Reese’s Pieces before baking.

Bake for 19-22 minutes or until edges are lightly browned. The center may appear too soft, but it will set as the cookie cools.

Cool the cookie on the baking sheet for 10 minutes before eating. As it cools, press a few Reese’s Pieces and/or chocolate chips into the top, if desired, for garnish. Cookie freezes well up to 3 months. Thaw at room temperature.