Combine melted butter and sugars in a large bowl and stir until well-combined.
1 cup unsalted butter, melted and cooled until no longer warm to the touch, 1 ¼ cup brown sugar* tightly packed, ½ cup sugar
Add egg, egg yolk, and vanilla extract and stir well.
1 large egg + 1 egg yolk room temperature preferred*, 1 ½ teaspoons vanilla extract
In a separate, medium-sized bowl, whisk together flour, cornstarch, baking soda, baking powder, and salt.
2 ¾ cups all-purpose flour, 1 Tablespoon cornstarch, ¾ teaspoon baking soda, ½ teaspoon baking powder, ¾ teaspoon salt
Gradually add dry ingredients to wet and mix until combined.
Use a spatula to stir M&M candies into the cookie dough
1 ½ cups M&Ms
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.
Video
Notes
Storing
Store in an airtight container at room temperature for up to 5 days.