Add 1 sachet kalamansi juice to reserve mango syrup; set aside. Heat a nonstick frying pan and melt 2 tablespoons butter. Add brown sugar into the foamy melted butter, stirring well with a wooden spoon. Add chopped mangoes, cooking for about 2 minutes. Pour in ¼ cup of rum. Carefully ignite the rum with a lighted wooden skewer or long-stick match; let the sauce flame until it dies out. Stir in the reserved mango syrup/kalamansi juice and keep cooking till of sauce consistency, stirring well constantly. Spoon sauce over scoops of vanilla ice cream for a fruity ice cream sundae. You may garnish with chopped nuts and whipped cream.