Preheat oven to 350 degrees.
Combine butter, sugar, and brown sugar in mixing bowl and use stand mixer to blend ingredients well.
Mix in the egg & vanilla.
In a separate bowl, combine flour, baking soda, baking powder, cornstarch, & salt.
Add dry ingredients slowly to mixing bowl and use stand mixer to blend well.
Stir one cup of white chocolate chips into batter.
Press batter into a non-stick sprayed 9x13 pan, using a butter knife to spread batter evenly in pan.
Bake cookie dough for 13 minutes.
Remove from oven and sprinkle marshmallows and M&M's on top.
Return to oven for approximately 4 minutes.
Remove and place onto cooling rack.
Sprinkle remaining white chocolate chips and sugar sprinkles on top of the bars.
Cool completely before cutting into bars.