In the bowl of a stand mixer fitted with the paddle attachment (or using a handheld mixer), cream butter, brown sugar, and granulated sugar until light in color, about 1 minute.

Scrape down the sides of the bowl and add egg, egg yolk, and vanilla extract. Mix until just combined, scraping down the sides of the bowl once in between.

Add the flour, cornstarch, baking soda, and kosher salt to the bowl. Slowly mix until the batter is smooth and the flour is thoroughly combined. Fold in the chocolate chips and mini Reese's Pieces.

Cover the dough with plastic wrap and chill in the refrigerator for at least 4 hours, or overnight.

Preheat the oven to 350°F. Spray a 12-cup muffin tin with nonstick baking spray.

Heat the peanut butter in the microwave for 20 seconds to soften.

Press 1 tablespoon of dough into the bottom of each muffin tin. Top each dough ball with 1 teaspoon of peanut butter. Roll an additional 1 tablespoon of dough into a ball and flatten into a fat disc using the palms of your hands. Place each disc directly on top of the peanut butter.

Bake for 16-17 minutes, or until light golden brown. Cool on a wire rack completely. Run a knife around the edges of each cookie to loosen. Gently pop each cookie out of the cup.