Preheat the oven to 350ºF and spray a 9-inch pie dish with nonstick cooking spray.

In a medium saucepan, melt the butter over medium-low heat. As the butter melts, it’ll start to foam, then turn a clear golden color. Continue to stir the butter, scraping up any browned specks from the bottom of the pan as it starts to brown. It should have a nutty aroma. Once browned and nutty, remove from the heat; set aside to cool slightly.

Whisk the flour, baking soda, salt, and cornstarch together in a medium bowl; set aside.

Transfer browned butter to a large bowl. Using a fork, vigorously stir in both sugars until smooth and combined, about 1 minute. Stir in the eggs and vanilla. Add the dry ingredients, stirring until just incorporated. Stir in 1/4 cup of the semi-sweet chips, 1/4 cup of the white chips, and 1/4 cup of the M&M’S® candies. Press dough into the pie dish, and place remaining chocolate chips and M&M’S® candies evenly into the dough.

Bake in the oven for 20-22 minutes, or until the edges are lightly browned and the center is set. Allow cookie pie to cool completely.

In a stand mixer fitted with the paddle attachment, cream the butter and salt together on high until smooth and creamy, about 1 minute. Add the powdered sugar, a cup at a time, scraping down the sides of the bowl with a silicone spatula as needed. Add the vanilla and 2 Tablespoons of milk, beating until light and fluffy. Transfer frosting to a piping bag with a piping tip.

Once cookie has completely cooled, pipe a border of buttercream around the edges of the cookie. Decorate with M&M’S® Chocolate Bar pieces and sprinkles. Slice, serve, and enjoy!