*Note: The size of the baking pan really depends on how thick you want your cookie bars. I baked mine in a 9×9 pan because it was what I had available. However, I do think a 9×13 pan works better. If you use a 9×13 pan (as written), yours will not be quite as thick.
2 1/8 cups flour
1/2 tsp salt
1/2 tsp of baking soda
12 Tbsp butter (1 1/2 sticks), melted and cooled slightly
1 cup of brown sugar
1/2 cup white sugar
1 large egg
1 large egg yolk
2 tsps vanilla extract
1 12oz bag M&M’s, divided
Preheat the oven to 325 degrees. Adjust the oven rack to lower-middle position. Grease a 9×13 pan and set aside.
In a medium sized bowl, whisk together the flour, salt, and baking soda and set aside.
In a large bowl, whisk the melted butter and sugars until combined. Add the egg, egg yolk, and vanilla and mix well. Using a rubber spatula, fold the dry ingredients into the egg mixture until just combined; do not over mix. Fold in 1 cup of M&M’s and turn the batter into the prepared pan smoothing the top with the spatula. You can also use your hands! Sprinkle remaining M&M’s on top and press in slightly.
Bake 25-30 minutes, until the top of the bars is light golden brown, slightly firm to the touch, and edges start pulling away from sides of pan. Cool completely before cutting.