Preheat oven to 325 degrees. Line an 9x13-inch metal baking pan with parchment paper, set aside.

In a microwave safe mixing bowl, add the butter. Melt on half powder for 1-2 minutes or until almost melted. Whisk to completely melt.

Add sugars, egg, egg yolk and vanilla extract. Beat mixture by hand for 30 seconds. Let the batter rest for 30 seconds then beat again for 30 seconds.

Add flour, salt, and baking soda. Using a rubber spatula, mix until combined. Fold in 1 cup of M&M's.

Transfer dough into the prepared baking pan. Using the spatula, spread the dough into an even layer. Sprinkle remaining 1/4 cup of M&M's on top.

Place in the oven and bake for 20-25 minutes or until the cookie bars are set. Remove from the oven and allow to cool in the pan for 20 minutes before cutting and serving.