Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
In a large bowl, cream together the butter, peanut butter, granulated sugar, and brown sugar until light and fluffy.
Beat in the vanilla extract and egg until well combined.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the M&M's.
Drop tablespoon-sized balls of dough onto the prepared baking sheet, spacing them about 2 inches apart.
Bake for 9-11 minutes, or until the edges are golden brown. Allow cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack.
Notes
Store cookies in an airtight container for up to a week. You can freeze the dough balls for up to three months and bake directly from frozen, adding a few extra minutes to the bake time.