Preheat oven to 350°F. Line a cookie sheet with parchment paper and set aside.
Add the butter, granulated sugar, and brown sugar to a large mixing bowl or large bowl of an electric stand mixer. Cream the ingredients together until light and fluffy, using a hand mixer or the paddle attachment on a stand mixer.
½ cup salted butter, ½ cup granulated sugar, 1 cup brown sugar
Add egg, egg yolk, and vanilla. Mix until well combined. Scrape down the side of the bowl as needed.
1 large egg, 1 egg yolk, 1½ teaspoons pure vanilla extract
Add flour, baking soda, baking powder, and salt, mixing until just combined.
2 cups all-purpose flour, 1 teaspoon baking soda, ½ teaspoon baking powder, ½ teaspoon salt
Stir in 1 cup of M&M's and chocolate chips.
½ cup milk chocolate chips, 1½ cups red and green M&M's
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 1/2 cup of the M&M's 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.