Place waffles on 4 serving plates. Slice bananas in half lengthwise, then crosswise so you have 4 pieces from each banana. Heat butter and sugar in a large skillet over medium heat, stirring, until sugar begins to dissolve. Place bananas in pan, cut side down, and cook until lightly golden, about 1 minute. Turn bananas and cook 1 minute longer. Top waffles with scoops of ice cream. Transfer bananas with a slotted spatula to plates and arrange around ice cream. Add rum to skillet and simmer 1 minute. Add cream and cook until sugar completely dissolves and sauce is thick, 1 to 2 minutes longer. Pour rum sauce over sundaes and serve immediately.