Combine flour, baking soda, cornstarch and salt. Set aside.
Cream butter and sugars. Add eggs and vanilla. Mix.
Gradually mix in dry ingredients.
Gently stir in M&M's and white chocolate chips.
Cover dough with Glad Press'n Seal and refrigerate overnight.
Preheat oven to 350°F.
Using 3½ Tb cookie scoop; drop dough on ungreased baking sheet.
Bake for 18-20 minutes or until edges just begin to brown. (Remove cookies slightly underbaked - Do Not Over Bake)
Allow to cool on pan for 10-15 minutes then transfer to cooling rack.