Preheat the oven to 350°F (175°C)
Line a baking sheet with parchment paper and set aside
In a stand mixer fitted with the paddle attachment, beat butter, granulated sugar, brown sugar, peanut butter, egg, and vanilla until fluffy
Slowly add the flour, baking soda, and salt, mixing until well combined
Fold in the M&M's candies
Drop tablespoon-sized scoops of dough onto the prepared baking sheet, spacing them evenly
Bake for 12 minutes until lightly browned
Immediately after removing from the oven, carefully press two pretzels into the top of each cookie
Transfer the cookies to a wire rack to cool completely