Preheat oven to 375 degrees. Use parchment paper or a silpat mat to line a baking sheet.
Cream the butter, sugar and brown sugar in a stand mixer until light and fluffy (about 2-3 minutes). Scrape the sides of the bowl to make sure all ingredients are blended.
Add in the egg and vanilla and beat until combined.
Add in flour, cornstarch, baking soda, and salt. Mix until flour is just incorporated. Try not to overmix.
Add in the chocolate chips, peanut butter chips and M&Ms and hand stir until it’s all combined.
Scoop out generous one tablespoon portions of cookie dough. I recommend a 1 TBSP cookie scoop. You can always make the cookies larger with 2 tablespoons of cookie dough per cookie but you will have to bake them a bit longer.
Bake the cookies at 375 degrees for 10 minutes. Remove from oven and immediately sprinkle with sea salt. Let cookies cool on the baking sheet for an additional 10 minutes. Remove cookies to cooling rack.