1. Preheat the oven to 325 degrees. Adjust the oven rack to lower-middle position. Line a 9X13-inch baking pan with foil, letting the excess hang over the edges of the pan, so you can grab those edges and pull the bars from the pan after they have baked. Spray the foil-lined pan with nonstick cooking spray. You could also use parchment paper and it would be fine.
2. Mix 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. 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 remaining M&M's on top and press in slightly.
3. 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, 24-28 minutes. Cool on a wire rack to room temperature. Remove the bars from the pan by lifting the foil overhang and transfer them to a cutting board. Cut into squares and serve.
