Preheat the oven to 350 and spray a 9×13 baking dish with pam baking spray. Using a hand mixer, beat together the butter, brown sugar, sugar, eggs, and vanilla until light and creamy.
It is important that you get the ingredients well aerated (lots of air in them) so that they bake properly. You want to see them go from a dark brown color to a light, creamy tanish color.
This makes a thick batter but one that you can still mix with a spoon. Fold in ½ cup M&M’s and all the sprinkles. Press the dough into the baking dish and spread it into an even layer.