Preheat your oven to 350°F (175°C).
Line a baking sheet with parchment paper.
In a large mixing bowl, combine the peanut butter, granulated sugar, egg, vanilla extract, baking soda, and salt. Mix until well combined and smooth.
Gently fold in the mini Reese's Pieces or chopped Reese's Peanut Butter Cups.
Using a cookie scoop or tablespoon, drop rounded balls of dough onto the prepared baking sheet, spacing them about 2 inches apart.
Flatten each ball slightly with the back of a fork, creating a crisscross pattern if desired.
Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden.
Remove from the oven and let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.