Preheat the oven to 350°F and line a baking sheet with parchment paper.
Cream together the butter, peanut butter, brown sugar, and granulated sugar. Add the egg and vanilla extract, mixing until combined.
Combine flour, baking powder, baking soda, corn starch, and salt, then gradually mix into the creamed mixture until just incorporated. Fold in the chopped Reese’s peanut butter cups.
Using a large cookie scoop, form 9 equally sized dough balls. Roll each ball between your hands, then gently flatten to about 1 inch thickness. Freeze the cookie dough discs for 15 minutes.
Bake in the preheated oven for 12-13 minutes.
While cookies are baking, melt peanut butter chips with oil in the microwave in 30-second increments, stirring in between each interval.
Generously top each hot cookie with the melted peanut butter chips mixture, spread it evenly, then sprinkle with additional chopped Reese’s cups.
Allow cookies to cool completely on the baking sheet before serving.