Preheat the oven to 350ºF.

Cut the butter into tablespoon sized pieces and place in a microwave safe bowl. Microwave for about 45 seconds - you want it to be about 75% melted - add more time if needed. Remove and let the butter sit for 5 minutes.

Once the butter has cooled slightly, place it in a large bowl or the bowl of a stand mixer. Add both the granulated sugar and the brown sugar. Mix together until the mixture is smooth and creamy.

Add the egg and vanilla and mix just until combined.

Add the flour, cornstarch, baking soda, and salt. Mix just until it starts to come together. The dough should not be too wet or too dry.

Reserve a few of the m&ms for topping the cookies and stir the rest into the dough by hand. You don’t want to overmix the dough.

Roll the dough into balls about 1 1/2 tablespoons each. If desired, roll the balls in holiday sprinkles, or press additional m&ms into the tops of the dough. Place the dough on a baking sheet lined with parchment paper or a silpat liner, at least 2 inches apart.

Bake the cookies just until the edges start to brown, about 10 minutes.

Remove from the oven and let the cookies sit on the baking sheet for about 10 minutes before removing to a sheet of parchment or a cooling rack.