Preheat the oven to 350℉/180℃. Line a baking sheet with parchment paper. Set aside.
In a large bowl, cream together the butter and sugars until well combined (2 minutes). Add in the egg and vanilla and continue creaming until paler in colour (Another 2 minutes).
3/4 cups butter, 3/4 cups brown sugar, 1/2 cup white sugar, 1 egg, 1 tbsp vanilla
Add flour, baking soda, salt, and cornstarch into the bowl and mix until just combined and no flour streaks appear.
2 1/4 cups all-purpose flour, 2 tsp cornstarch, 1 tsp baking soda, 1/2 tsp salt
Toss in the M&M's and chocolate chips and mix until just incorporated. Don't overmix!
3/4 cups spring M&M's, 3/4 cups semi-sweet chocolate chips
Scoop out about 2 tablespoons of cookie dough and roll into balls. An ice cream scooper works best for this.
Bake 6 cookies at a time for 12-15 minutes. The middle will look under-baked, we want it this way!
Gently press additional M&M's and chocolate chips on top of the cookies if desired.
Let the cookies sit for several minutes on the baking sheet before transferring to a cooling rack. This will help them continue to bake before cooling completely.
Enjoy!