Line a 9 x13 baking pan with greased parchment paper. Set aside.
In a medium bowl, stir together flour, salt and baking soda. Set aside.
In a large bowl, cream together melted butter, brown sugar, and white sugar.
Stir in large egg and egg yolk until combined.
Stir in vanilla extract.
Combine dry ingredients with wet ingredients, stirring just until flour is incorporated. Don't over stir.
Add in 1 cup of m&m's.
Press dough into an even layer in prepared pan.
Press remaining m&m's lightly on the top.
Bake for 25-30 minutes.
Remove from pan and let cool completely
Cut into squares.
I greased my parchment paper with butter-flavored shortening. This gave a beautiful, golden, crisp, buttery tasting bottom. Be sure to leave enough overhang on the parchment to make it easy to remove from the pan when done.