Preheat oven to 350°F. Spray a 9-inch cast iron skillet with cooking spray.
Add butter to a large bowl with sugars. Stir to combine. Stir in egg yolk, extract, baking soda, and salt until combined.
Add flour and stir to combine. Fold in M&M's and chocolate chips.
Pour batter into the greased skillet. Bake for about 16-18 minutes. The edges should be olden and puffed, but the center should be soft.
Serve with vanilla bean ice cream, hot fudge sauce, and Valentine's Day theme sprinkles.