In a large mixing bowl, cream together the butter and sugars until light and fluffy (about 1 minute). Beat in the vanilla and eggs. Stir in the flour, baking powder, baking soda and salt. Fold in 2 cups of the M&M's.
Using a large rubber spatula, spread dough evenly into baking sheet. The dough may be thin if using a very large sheet pan, but will puff and close holes during baking. Sprinkle additional ½ cup of M&M's over top and lightly press into the dough.
Bake for 20 to 30 minutes*, until baked to desired doneness and edges are golden. Do not over bake. Let cool.