Combine your melted (and cooled) butter and sugars in a large bowl and stir until well-combined.
Add egg, egg yolk, and vanilla extract and stir until combined.
In a separate, medium-sized bowl, whisk together flour, cornstarch, baking soda, baking powder, and salt.
Gradually add dry ingredients to wet and mix until combined.
Use a spatula to stir M&M candies into the cookie dough
Cover bowl with plastic wrap and transfer to the refrigerator to chill for at least 45 minutes.
Preheat oven to 350F (175C) and line a cookie sheet with parchment paper.
Once oven has preheated, remove your chilled cookie dough from the fridge and roll dough into approximately 2-Tablespoon-sized scoops. Roll gently to form a ball and place on prepared cookie sheet (space cookies at least 2" apart).
Bake on 350F (175C) for 10-11 minutes, centers of the cookies may still seem slightly underbaked -- allow cookies to cool completely on cookie sheet to finish cooking there, this will help keep them nice and soft.