Preheat the oven to 350°F (180°C).
Combine the butter, peanut butter, and sugars until light and fluffy.
½ cup (111g) salted butter, softened, ½ cup (123g) creamy peanut butter, ¾ cup (185g) brown sugar, ¼ cup (58g) granulated sugar
Add in the egg, milk, and vanilla. Mix until well combined.
1 egg, 2 Tbsp milk, 2 tsp vanilla extract
Add the flour, rolled oats, baking soda, and salt to the mixer and mix until a soft dough comes together.
1 ½ cups (228g) all-purpose flour, ¾ cup (75g) rolled oats, 1 tsp baking soda, ½ tsp salt
Add in the Reese’s Cups and peanuts, mixing gently to distribute throughout the dough.
1 cup (200g) Reese’s Cups, chopped, ¾ cup (94g) cocktail peanuts
Portion the dough into 3-tablespoon balls (#24 scoop). Break the dough ball in half, flip the halves so the rough interior is on top and push the pieces back together to create a dough ball with a textured top. (Not necessary, but fun!)
Arrange cookies on a parchment-lined cookie sheet about 2 inches apart and bake for 8–10 minutes until the edges begin to brown.
Let the cookies cool for 5–10 minutes on the pan before transferring to a wire cooling rack. Store in an airtight container at room temperature for up to 5 days.