Brown the Butter: Place the butter in a medium saucepan over medium heat. Melt it completely, then whisk in the milk powder until fully combined. Increase the heat slightly and cook, whisking constantly, until the butter turns golden brown and has a nutty aroma (about 5-8 minutes).
1 cup 2 sticks; (227 g) unsalted butter, 1/4 cup (28 g) low fat milk powder
Stir in Water: Remove the pan from heat and pour the browned butter into a large mixing bowl. Stir in the cold water to cool it slightly.
2 Tablespoons cold water
Mix Ingredients: Add the brown sugar, granulated sugar, and vanilla extract to the browned butter. Whisk until smooth and combined. At this stage, the mixture should be cool enough to add the eggs without cooking them. If it still feels warm, simply place it in the freezer for a few minutes to bring it down to room temperature.
1 cup (200 g) brown sugar, 2/3 cup (133 g) sugar, 1 Tablespoon vanilla extract
Add Eggs: Add the eggs and egg yolk, one at a time, mixing well after each addition.
2 large eggs, plus one egg yolk
Prepare the Dry Ingredients: In a separate bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
3 cups (390 g) all purpose flour, 2 teaspoons cornstarch, 1 teaspoon baking powder, 1 teaspoon baking soda, 1 teaspoon Morton kosher salt*
Fold in the Mix-Ins: Stir in the chocolate chips, M&Ms, and Christmas sprinkles until evenly distributed.
1 cup (6 ounces) semi sweet chocolate chips, 3/4 cup M&Ms, plus more for topping, 1/3 cup Christmas sprinkles, plus more for topping
Shape and Chill: Use a 3-tablespoon cookie scoop to portion the dough. If the dough feels too sticky to shape, place it in the freezer for a few minutes until it firms up and becomes easier to handle. Roll each portion into a ball, then shape it so it’s taller than it is wide (like a cylinder). Place the shaped dough balls on a parchment-lined baking sheet. Freeze the cookie dough balls for 20-30 minutes or until firm. After freezing, transfer the dough balls to an airtight container or cover them on the baking sheet and store them in the refrigerator for up to 2-3 days if you are not baking them right away.
Preheat Oven & Arrange: Preheat your oven to 350°F (175°C). Arrange the chilled cookie dough balls on a parchment-lined baking sheet, spacing them about 2 inches apart (6-8 per standard size baking sheet) Press extra M&Ms and sprinkles on top for decoration.
Bake & Enjoy: Bake for 12-14 minutes, or until the edges are set and the centers look slightly underbaked. Let the cookies cool on the baking sheet for at least 5 minutes before transferring them to a wire rack to cool completely.