Preheat oven to 375°F.
In a mixing bowl, add butter, peanut butter, sugar, and brown sugar. Using an electric hand mixer, cream these together until light and fluffy (2-3 minutes).
Add egg and vanilla.
Once mixed add flour, salt, and baking soda.
The cookie dough will look crumbly but stick together when scooped and rolled into a ball. Scoop and roll dough into 20 balls. If using a potion scoop, use size 40 or D40.
Roll each ball in granulated sugar.
Place 10-12 dough balls on a parchment-lined cookie sheet.
Bake at 375ºF for 9-10 minutes. Remove and immediately press a few M&Ms into each cookie, I did 5 M&Ms per cookie.
Bake for additional 1-2 minutes to heat M&Ms.
Cool and enjoy.