Preheat oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a large bowl, beat together peanut butter, softened butter, granulated sugar, and brown sugar until creamy.
Add the egg and vanilla extract, mixing well.
In a separate bowl, whisk together flour, baking soda, baking powder, and salt.
Slowly add the dry ingredients to the wet mixture, stirring until just combined.
Fold in Reese’s pieces or chopped Reese’s cups.
Scoop dough into balls (about 1 tablespoon each) and place on baking sheet.
Bake for 10–12 minutes, until edges are lightly golden.
Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack.