Melt and cool (for at least 10 minutes) the butter.
Stir together the melted/cooled butter, granulated sugar and brown sugar until well combined.
Add the room temperature egg, egg yolk and vanilla extract to the butter/sugar mixture. Stir until well combined.
In a separate bowl, whisk together the flour, baking soda, baking powder, corn starch and salt.
Add the whisked dry ingredients to the wet ingredients and mix until just combined.
Gently fold the M&M’s into the mixed cookie dough.
Cover the dough with plastic wrap and let chill in the refrigerator for 45 minutes.
When the dough is almost done chilling, preheat oven to 350F. Line the cookie sheet with parchment paper. Roll the dough into approximately a couple of tablespoon sized scoops. Roll into a ball. Place on the cookie sheet and sprinkle with sugar crystals to taste.
Bake at 350F for 10-12 minutes. The centers of the cookies may seem a little underbaked, but allow them to finish cooling on the cookie sheet. They will finish baking while they are cooling.