Preheat oven to 350°F. and line a couple baking sheets with parchment paper.
Mix dry ingredients together: In a large bowl, whisk together the all-purpose flour, cornstarch, baking soda, baking powder and salt. Set aside.
2 cups all purpose flour, spooned and leveled, 1 tsp. corn starch, 1/2 tsp. baking soda, 1/2 tsp. baking powder, 1/2 tsp. salt
Cream the butter with the sugars using the paddle attachment of a stand mixer, for 2 minutes, scraping down the sides of the bowl as needed. It will sort of look like a paste, not really creamed since our butter is cold.
1/2 cup unsalted butter, cold, cut into cubes, 1/2 cup brown sugar, packed (light or dark), 1/4 cup granulated sugar
Then add in the egg, egg yolk and vanilla extract. Mix until combined.
1 large egg, room temperature, 1 large egg yolk, room temperature, 1 tsp. vanilla extract
Gradually add in the flour mixture until ingredients are incorporated.
Fold in the chocolate chips and M&Ms using a rubber spatula.
1 cup chocolate chips, 3/4 cup M&Ms (Cupids Mix for Valentine's Day)
Scoop the cookie dough using a 2 Tbsp. cookie scoop onto the prepared baking sheet.
Place extra M&M’s and chocolate chips on the top of the cookies if desired (I always like to do this, makes them look cuter!)
Bake for 9-11 minutes, (I like to go slightly underbaked for a delicious chewy cookie, the edges should be a light golden brown and centers still a little gooey looking, they will continue to bake on baking sheet when removed from oven).
Remove from oven, gently press any additional M&M candies and chocolate chips on the cookies if you want while they're still warm and let the cookies rest on the cookie sheet for just 1-2 minutes, then transfer to a cooling rack, garnish with flaky sea salt and enjoy!
flaky sea salt for finishing, optional