Preheat the oven to 350°F. Lightly grease (or line with parchment) two baking sheets.
Using mixer, beat together butter, sugars, egg, vanilla, and peanut butter until smooth.
Add the flour, baking soda, and salt to the peanut butter mixture, beating gently until everything is well combined and fold in the mini M&Ms.
Drop the cookie dough by tablespoonful or by using a 1 1/2 Tablespoon cookie scoop, onto the prepared baking sheets leaving 2" in between each cookie.
Gently use a fork to flatten each cookie to about 1/2" thick, making a cross-hatch design.
Bake the cookies for 12 to 16 minutes, until they're barely beginning to brown around the edges to get best results for soft and chewy peanut butter cookies. Bake longer for crunchy. Remove them from the oven and let set to cool on baking sheet for 5 minutes to set up so they don't fall apart.