Preheat oven to 350 degrees. Line a cookie sheet with parchment paper and set aside.
Using a stand mixer, or a hand mixer, cream together butter, sugar, and brown sugar, until light and fluffy.
Add egg, egg yolk, and vanilla. Mix until well combined. Scrap down the side of the bowl as needed.
In a medium mixing bowl, whisk flour, baking soda, baking powder and salt. Slowly add dry ingredients to cookie batter, and mix until just combined.
Using a rubber spatula, or a wooden spoon, stir in 1 cup of M&Ms and chocolate chips.
Use a medium-sized cookie scoop, and scoop out the dough into balls, place them on the cookie sheet 1 inch apart. Use the remaining ¼ cup of the M&Ms and press them on top of the dough balls.
Bake for 10-12 min. Remove from the oven and allow to cool on a cooling rack for 5 min.
Notes
These cookies can be stored in an airtight container at room temperature for up to 1 week. You can also place them in an airtight container in the freezer for up to 3 months, once they have cooled.