Preheat oven to 325F degrees. Line a large cookie sheet with a silicone baking mat or parchment paper. Set aside.
In a large mixing bowl, stir the butter, peanut butter and sugars together until creamed. Add egg and vanilla, mixing until well combined. Stir in flour, oats, baking soda and salt. Fold in the chocolate chips and 1/4 cup M&Ms until combined. With your hands, scoop dough onto prepared baking sheet. Shape dough into a big cookie, about 6 inches wide. Press the M&M's into the top of the cookie.
Bake for 20-22 minutes, until edges are lightly browned. DO NOT OVER BAKE! The center may appear undone, but it will set as the cookie cools. Let cool on baking sheet for 15 minutes.
Cut into 4 pie shaped pieces, if desired.
Recipe adapted from Sally's Baking Addiction