Adjust the oven rack to lower-middle position, then preheat the oven to 325 degrees. Line a 9X13-inch baking pan with foil or parchment paper for easy removal. Spray with nonstick cooking spray.
Whisk together the flour, salt, and baking soda together in medium bowl; set aside.
Whisk the melted butter and sugars in a large bowl until combined.
Add the egg, egg yolk, and vanilla and mix well.
Fold the dry ingredients into the egg mixture until just combined, by not overmixing.
Fold in most of the M&M's and turn the batter into the prepared pan, smoothing out the cookie dough evenly.
Sprinkle remaining M&M's on top and lightly press into dough.
Bake until the top of the bars is light golden brown, slightly firm to the touch, and edges start pulling away from sides of pan, 30-35 minutes.
Cool on a wire rack to room temperature.
Remove the bars from the pan by removing bars from pan & transfer them to a cutting board.
Cut into squares, serve.