Pre-heat oven to 350 degrees (F). Prep your cookie sheets by either layering them with silicone baking mats, or use parchment paper.
In a mixer, combine butter with your granulated sugar and brown sugar. Combine well.
Crack your egg into the batch and mix in the egg with the vanilla extract. Ensure it's mixing evenly and scrape the bowl as needed.
In a separate large bowl, mix flour with salt, cornstarch, and baking soda. Mix well. Slowly add in this dry ingredient mix into your mixer bowl, and whisk well as you add more ingredients.
Once majority of the ingredients have been mixed really well, slowly pour in your M&M's® Holiday Milk Chocolate and baking chocolate chips.
Using a cookie scoop or large spoon, place cookie dough balls onto baking mat/parchment paper, and leave about 1 1/2 inches in-between the cookies. Feel free to use a few extra M&M's® Holiday Milk Chocolate and place them on top of your cookie dough balls, so you will be able to see the candy on the tops of the cookies.
Bake for 10 minutes, or until the edges have started to lightly brown and harden.
Cool your cookies on a cooling rack, or remove them from the baking sheet with a spatula to a flat room-temperature surface.
Serve warm or cooled! Whatever you prefer--pair these cookies with Great Value 2% Milk, because it tastes awesome!