Preheat oven to 350 and grease a 9x13 baking pan with nonstick cooking spray and set aside.
Add brownie mix in a mixing bowl and prepare brownies according to package directions. Pour into the prepared pan.
In a separate bowl, add melted butter, brown sugar, oatmeal and peanuts. Mix well. Fold in 3/4 of the candies. Sprinkle mixture on top of brownies. Add remaining candies to the top of the brownies.
Bake in preheated oven for 40-45 minutes or until the center is set and edges begin to pull away from the pan.
Let cool before cutting into 16 squares.
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Notes
Feel free to use any color or variety of M&M's that you like. Be sure to purchase a package of 'family size' brownies meant for a 9x13 baking dish or your brownies will be very thin. You may sub in our homemade brownies, but bake time will increase.