Sprinkle sugar into a 10-inch cast-iron skillet. Cook over medium heat, stirring constantly with a wooden spoon, until sugar melts and turns light brown.
Remove from heat; add butter. Stir until butter melts.
Return to low heat and cook, stirring constantly, until mixture is well blended.
Combine whipping cream and milk.
Add whipping cream mixture to hot sugar mixture, 1 tablespoon at a time, stirring constantly, 25 minutes or until mixture is smooth and creamy.
