Prepare grill. Peel bananas and halve lengthwise. In a shallow baking pan large enough to hold bananas in one layer stir together butter and brown sugar and add bananas, tossing gently to coat. Transfer bananas with a metal spatula to an oiled rack set 5 to 6 inches over glowing coals and grill until browned and cooked through, about 2 minutes on each side. (If bananas are difficult to handle you may grill them on 1 side only.) Serve bananas like a banana split with ice cream and chocolate sauce.

To make rum ice cream:
In a 2 1/2- to 3 quart heavy saucepan combine cream and milk. Scrape seeds from vanilla bean into cream mixture, dropping in bean. Add zest and bring mixture just to boil. In a large metal bowl whisk together brown sugar and yolks until combined well. Add hot cream mixture to yolk mixture in a slow stream, whisking, and return custard to pan. Cook custard over moderately low heat, whisking constantly, until thickened and coats back of a wooden spoon, about 3 minutes. (Do not let custard boil or it will curdle.) Pour custard through a very fine sieve into cleaned metal bowl and cool slightly. Stir in rum and chill, covered, until cold. Freeze custard in an ice-cream maker. Makes about 1 1/2 quarts.

To make Mexican hot chocolate sauce:
In a heavy saucepan heat chocolate and half-and-half over low heat, stirring frequently, until chocolate is melted. (The sauce will have a very thin consistency, like that of hot chocolate.) Serve sauce warm. Makes about 1 cup.

Serves 4.