Preheat oven to 325°F. Grease a 12-inch cast iron skillet
Place butter and chopped chocolate in a medium heatproof bowl. Melt together by placing bowl on top of a saucepan with a few inches of barely simmering water, stirring often. * Keep a close eye on the chocolate as milk chocolate can burn quickly. Stir constantly once it starts to melt and remove from heat as soon as all ingredients have melted together. Set aside to cool to room temperature.
Tip batter into prepared cast iron skillet. Bake until top is set, and a skewer inserted into the center comes out clean, about 30-40 minutes. Let cool completely in pan before topping with ice cream.