Preheat oven to 325-degrees. Line a 9x13-inch baking pan with foil and spray the bottom and sides with baking spray. Set aside. In the bowl of a stand mixer, affixed with the paddle attachment, add the butter, brown sugar and white sugar. Mix on medium-high speed until light and fluffy. Change the mixer to low speed and add in the egg and egg yolk, vanilla extract and salt. Mix on medium high speed until well combined and creamy. Stop the mixer and scrape down the sides and bottom of the bowl.

On low speed, add the flour, baking soda and cornstarch, mixing until barely combined. Remove the bowl form the mixer and use a rubber scraper to finish incorporating all of the ingredients together. Gently fold in the candy bar pieces and M&M's into the dough.

Pour cookie dough into an even layer in prepared 9x13-inch pan. Bake in 325-degree oven for 20-22 minutes, or until the edges are just lightly brown. Do not over bake! Cookie bars may look under done, but they continue to bake and set in the pan after you remove them from the oven.

Allow cookie bars to set in the pan for 30-60 minutes before cutting into desired sized squares.