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1
Divide the grapefruit sections and the banana slices among four 6-inch (1- to 1 1/4-cup-capacity) gratin dishes.
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2
In a small saucepan combine the milk and the vanilla bean and bring the milk just to a boil.
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3
While the milk is heating, in a metal bowl whisk together the whole egg, the egg yolk, the granulated sugar, the flour, sifted, and a pinch of salt.
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4
Add the milk mixture in a stream, whisking.
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5
Transfer the pastry cream to the pan, bring it to a boil, and simmer it, whisking, for 2 minutes.
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6
Force the pastry cream through a fine sieve into a bowl, discarding the vanilla bean, whisk in the rum, and let the pastry cream cool slightly.
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7
In another bowl with an electric mixer beat the heavy cream until it holds stiff peaks, whisk a small amount of the whipped cream into the pastry cream, and fold in the remaining whipped cream gently but thoroughly.
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8
Pour one forth of the cream mixture into each gratin dish, spreading it to cover the fruit completely, and sprinkle each gratin with some of the confectioners' sugar.
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9
Arrange the gratins in a large baking pan and broil them under a preheated broiler about 4 inches from the heat for 2 to 4 minutes, or until they are browned and heated through.
