In a large skillet, melt butter over medium heat. Add plums and sprinkle with sugar. Saute, shaking skillet occasionally, until sugar has melted and begun to caramelize, about 5 minutes. Add Cointreau and orange juice. Remove skillet from heat. Carefully ignite, and gently swirl pan to loosen the cooked sugar and plums. Return to heat and cook until flame has extinguished and liquid is reduced by half about 3 minutes.