HEAT oven to 325ºF. Melt 1/4 cup butter in 10-inch cast iron skillet. Remove from heat; stir in 1/2 cup sugar and pineapple; spread evenly.
COMBINE mix, nutmeg, 1/4 cup sugar, milk, egg, 1/2 cup butter and sour cream in medium bowl. Stir just until blended. Pour batter over pineapple in skillet.
BAKE 35 to 40 minutes or until golden brown. Remove from oven. Loosen edges of cake. Place serving plate on top of skillet; carefully invert cake onto plate. Serve topped with whipped cream, if desired.