Preheat oven to 350°F. Lightly grease your baking sheet or line with parchment paper.
In a large bowl, combine softened butter, granulated sugar, and brown sugar. Cream together until light and fluffy (2–3 minutes).
Mix in eggs, vanilla extract, and peanut butter until fully incorporated.
In a separate bowl, whisk together flour, baking soda, and salt. Slowly mix this into the wet ingredients until well combined.
Gently fold in the Reese’s Pieces.
Scoop the dough into heaping tablespoon-sized balls (about 1.5 tablespoons each). Roll gently in your hands to shape.
Place cookies a few inches apart on the baking sheet.
Bake for 10–12 minutes. Start checking around 8 minutes—edges should be lightly golden, and centers still slightly soft.