Place ¾ cup unsalted butter, in a small saucepan over medium heat and heat until golden brown specks appear, swirling the pan occasionally. Butter will get foamy and if you swirl the pan around, you will see brown bits at the bottom and it will have a nutty smell.
Remove the butter from heat and pour into a small bowl. Make sure to get all those browned bits at the bottom of the pan, that is where all the flavor is. Cool the butter until it's solid again.
When the brown butter is solid, preheat oven to 350 degrees Fahrenheit and line a baking sheet with parchment paper.
Using a hand mixer, whisk together the brown butter, ¾ cup packed light brown sugar, and ¼ cup sugar until light and fluffy. Add in the 1 large egg, 2 teaspoons vanilla extract, and 1 ½ tablespoons low-fat milk and beat until combined.
In a separate bowl, combine the 2 cups all-purpose flour, 1 teaspoon cornstarch, 1 teaspoon baking soda, and ½ teaspoon salt. Mix the flour mixture with the butter mixture until just combined. Stir in 1 cup M&M's, Chill dough for at least one hour or overnight.
Roll the dough into balls, around 1 1/2 Tablespoons of dough per cookie, and place them on the prepared baking sheet. Press a few extra M&M's onto the top of each dough ball.
Bake for 7- 9 minutes. Cookies will look underdone but set up perfectly once they cool. Do not overbake or the cookies will be dry.