Buttered Rum Mug Cake . Tip: “Be sure to use unsalted butter, since the self-rising flour has salt in it already. For extra rum flavor, use rum extract in place of vanilla.” 4 tablespoons self-rising flour 4 tablespoons dark brown sugar Pinch of ground nutmeg 2 tablespoons beaten egg or liquid egg substitute 1 tablespoon dark rum 2 tablespoons milk 2 tablespoons melted unsalted butter 1/2 teaspoon vanilla or rum extract Mix the flour, sugar, and nutmeg in a mug. Add the egg, rum, milk, butter, and vanilla and stir until well combined. Microwave on high for 1 minute or until the top is just dry.