In a large (10 inch) cast iron skillet, melt the butter.
Add the brown sugar and granulated sugar and stir until dissolved.
Allow this to cool down until it is just warm. This will keep the eggs from scrambling.
Add 2 beaten eggs.
Stir in 2 teaspoons vanilla.
Mix the flour, baking soda and salt with a whisk.
Stir in the flour mixture with the butter and sugar mixture. The batter will become thick.
Stir in the Mega M & M's.
Press the batter down into the skillet.
Bake for 25 to 30 minutes at 375 degrees.