preheat the oven to 325°F. adjust the oven rack to lower-middle position. grease a 9×13 pan + set aside.

in a medium sized bowl, whisk together the flour, salt, + baking soda; set aside.

in a large bowl, beat the butter + sugars until light + fluffy. add the egg, egg yolk, + vanilla + mix well. using a rubber spatula, fold the dry ingredients into the egg mixture until just combined; do not overmix.

fold in 1 cup of M&M’s + turn the batter into the prepared pan, smoothing the top with the spatula. sprinkle the remaining M&M’s on top + press in slightly.

bake 25-30 minutes, until the top of the bars is light golden brown, slightly firm to the touch, + edges start pulling away from sides of pan. cool completely before cutting.