Preheat oven to 350 degrees Fahrenheit. Line a baking sheet with parchment paper or grease. Set aside.
In a large bowl or stand mixer, cream together butter and sugars until it becomes light and fluffy- around 5 minutes. Mix in egg, egg yolk, vanilla extract, and peanut butter until smooth.
In a separate bowl, whisk together flour, oats, baking soda, baking powder, cornstarch, and salt. Add dry ingredients to wet ingredients and mix until blended.
Fold in the semi-sweet chocolate chips, white chocolate chips, and 1/2 cup of M&M's.
Scoop dough using a heaping tablespoon or cookie scoop and place onto the cookie sheet around 2 inches apart.
Bake for 9-11 minutes or until the edges are just slightly golden and the center looks just finished (it will continue to cook on the baking sheet). Remove from oven and top with additional M&M's. Allow to cool for 10 minutes on the baking sheet before removing to a cooling rack to finish cooling. Enjoy!