Heat oven to 400° F. Peel and cut each banana lengthwise in half. Place the bananas in a single layer, slightly overlapping, in a 9-by-13-inch baking dish. Pour the lemon juice over the bananas; sprinkle with the sugar, liqueur, and butter. Bake for 15 minutes, spoon the juices over the bananas, and continue baking until soft and lightly browned, about 5 to 10 more minutes.
To serve, divide the bananas among 6 shallow bowls or plates, top with small scoops of ice cream, and garnish with chocolate shavings.