Step 1

Preheat oven to 350°F. Line a 9 inch pie dish with a round of parchment paper and lightly grease the dish. Set aside.

Step 2

With a hand mixer or stand mixer, beat butter and sugars on medium speed for about one minute, until smooth and creamy. Add egg and vanilla extract, and beat to combine, scraping the sides of the bowl as necessary.

Step 3

In a separate bowl, combine flour, cornstarch, baking soda, and salt. With the mixer on low, slowly add the flour mixture to the wet ingredients, beating until combined (about a minute). By hand, stir in chocolate chips and M&M's (if desired you can reserve a few M&M's to press onto the top of the cookie).

Step 4

Evenly press the dough into the prepared pie dish. The dough will be thick and sticky. Wet your fingertips with a little water (if necessary) to help flatten the dough into the dish. Lightly press the reserved M&M's on top.

Step 5

Bake for 20-25 minutes, until the edges are beginning to turn light golden brown and the very center is still a bit underdone. Place on a wire rack to cool completely.

Step 6

With a stand mixer or hand mixer, beat butter, powdered sugar, and cocoa powder (if using) on medium speed. Add vanilla extract and 1 tablespoon milk. Beat for an additional minute, adding a bit more milk until you reach your desired consistency. If needed, you can add more powdered sugar to thicken.

Step 7

Use a decorating tip (I used a star tip) to frost the outer edge of the cookie. Decorate with M&M's and sprinkles.