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; set aside.
In a large bowl, beat the butter and sugars until light and fluffy.
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 overmix.
Fold in 1 cup of M&M’s and turn the batter into the prepared pan, smoothing the top with the spatula.
Sprinkle the 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.