Melt butter in a skillet set over medium high-heat that's large enough to hold the bananas in a single layer. Sprinkle in sugar and cook until it melts and becomes bubbly. Add orange juice and rum - be careful of splatters from the pan - and simmer sauce until it lightly thickens. Add pecans and bananas and cook until bananas just heat through and become nicely coated with sauce. Spoon bananas over ice cream and serve immediately. Garnish with mint sprigs if desired.