About 1 3/4 cups sweetened condensed milk
2 tablespoons cornstarch
3 eggs, separated
Granulated sugar, to taste
Juice of 1 lime or lemon
Few drops of vanilla extract
Add enough water to the condensed milk to make 1 quart. Put the cornstarch in a cup and stir in a little milk until smooth.
In a heavy-based saucepan, bring the remaining milk to a boil. Pour into the blended cornstarch, stirring. Return the mixture to the boil again, stirring constantly. Continue to cook until the mixture thickens. Check for sweetness.
Beat the egg yolks, with sugar if needed, in a bowl and stir in the hot milk. Mash the bananas with the lime or lemon juice and beat into the custard with the vanilla extract. Leave to cool, stirring occasionally. Pour the mixture into a container, cover and freeze until just becoming firm. Beat well in a bowl. Whip the egg whites until stiff, but not dry, and fold into the custard. Spoon the mixture back into the container. Cover and freeze until firm.
About 30 minutes before serving, transfer the ice cream to the refrigerator.
Serves 6 to 8.