In a bowl, combine the apple with 1 tablespoon of the sugar and the cinnamon. In a large heavy skillet, melt the butter over moderately high heat. Add the apple and cook over high heat, stirring, until lightly crusted and tender but not mushy, 3 to 5 minutes. Add the rum, carefully ignite it and stir the apple to coat. Transfer to a plate and let cool to room temperature.