In a large skillet, cook the butter over high heat until lightly browned, 1 to 2 minutes. Stir in the brown sugar and cinnamon. Add the pineapple and cook over moderate heat, shaking the skillet, until the sugar is melted and the pineapple is slightly tender, about 2 minutes. Flip the pineapple spears. Add the rum, carefully ignite and cook until the flames subside. Transfer to plates and serve with ice cream.