Set your oven to 350°F (175°C). Line a baking sheet with parchment paper or a silicone mat for easy cleanup and even baking.
In a large bowl, beat softened butter, granulated sugar, and brown sugar with an electric mixer for 3–4 minutes until light and fluffy.
Mix in the peanut butter, egg, and vanilla extract. Beat until smooth and fully combined.
In another bowl, whisk together flour, baking soda, and salt. Slowly add the dry mixture to the wet ingredients, mixing just until combined.
Gently stir in chocolate chips and chopped Reese’s Peanut Butter Cups until evenly spread throughout the dough.
Use a small cookie scoop or tablespoon to form 1-inch dough balls. Place them 2 inches apart on the prepared baking sheet.
Bake for 8–10 minutes, until edges are lightly golden and centers still look soft—they’ll set as they cool.
Let the cookies cool on the baking sheet for 5 minutes, then move them to a wire rack to cool completely.