Preheat the oven to 350°F (177°C). Line a sheet pan with parchment paper and set aside.

Combine the cooled, melted butter, brown sugar, and granulated sugar in a medium mixing bowl. Whisk until the mixture is pale and thickened, 1-2 minutes. Then, mix in the egg yolk and vanilla extract until smooth.

Add the flour, cornstarch, baking soda, and salt. Using a rubber spatula, fold the dry ingredients into the wet ingredients until the dough is about 90% combined. Then, add the M&Ms and continue folding the dough just until everything is well-mixed and there are no more streaks of flour. Don't over-mix or the cookies will come out dense and crumbly!

Using a #20 cookie scoop (~3 Tablespoons), divide the dough into 6 even balls of dough. Line the cookie dough balls on the prepared sheet pan about 3 inches apart. Freeze the cookie dough balls for 15 minutes, or refrigerate for 30 minutes, until the dough is well-chilled.

Bake the cookies for 9-11 minutes, flipping the pan halfway through, until the edges are set and the centers are still a bit under-done. Right after the cookies come out of the oven, press a few more M&Ms to the top for decoration, if desired. Let the cookies cool for 5 minutes on the sheet pan and then transfer them to a wire rack to cool completely. Enjoy!