Preheat the oven to 350°F. Line large baking sheets with silicone baking mats or parchment paper.

Combine flour, baking soda, and salt in a medium mixing bowl and whisk together.

In a large mixing bowl, cream the melted butter, peanut butter, brown sugar, and granulated sugar until smooth.

Add the egg and vanilla extract to the butter and sugar mixture and beat until well combined.

Add the dry ingredients to the creamed mixture and mix until just combined, then stir in Reese's Pieces candies.

Using a large cookie scoop or spoon, scoop out a heaping tablespoon of dough and roll into smooth balls. Place on baking sheet at least 2 inches apart.

Gently flatten each ball slightly with the bottom of a glass. Bake for 10 minutes or until edges turn golden brown but centers are soft.

Remove cookies from oven and press additional Reese's Pieces into each cookie while still hot.

Allow cookies to cool on the baking sheet for 10 minutes, then transfer to a wire rack to cool completely.