In a bowl, cream or mix melted butter, sugars and vanilla until fluffy. If using a stand mixer, about two minutes on medium.
Add egg and egg yolk. Mix to combine.
Add dry ingredients-flour through baking soda and stir until flour has just been incorporated. Don't over mix.
Add white chocolate and 1 cup of the M&M's candies. Stir into the dough until combined.
Preheat oven to 350 degrees.
Spray 13 x 9 pan with non-stick spray and line with parchment paper.
Scoop dough into pan. Using your hands or a sheet of was paper, flatten dough until it covers the bottom of the pan.
Sprinkle remaining 1/4 cup of M&M's over the top of the mix and press the candy firmly into the dough.
Bake at 350 for 25 minutes or until golden brown. Cool on a cookie rack.
Slice into bars or triangles. If slicing into bars, you should get approximately 36, slicing six rows horizontally and then six rows vertically. But, you made this pan of M&M's cookie bars, you slice 'em however big or small you want 'em.