Combine melted butter and sugars in a large bowl and stir until well-combined.

Add egg, egg yolk, and vanilla extract and stir well.

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.