Preheat oven to 350° F and line a 13x9 pan with parchment.
Combine melted butter and sugars in a large bowl and stir well.
Add eggs, egg yolk, and vanilla extract and stir until completely combined. Set aside.
In a separate bowl, whisk together flour, cream of tartar, baking powder, and salt.
Gradually stir dry ingredients into wet until completely combined.
Fold in M&Ms.
Spread blondie batter into prepared pan and transfer to oven. Bake for 25-30 minutes or until a toothpick inserted in the center comes out clean or with a few fudgy crumbs. Allow to cool and enjoy!