In a small saucepan, bring the milk to a boil with the split vanilla bean, if using. Remove the pan from the heat, cover, and leave to infuse about 15 minutes.
In a small bowl, whisk the whole egg, 1 of the egg yolks, and the sugar until light and fluffy, about 3 minutes. Whisk in the cornstarch. Bring the infused milk just back to a boil. Pour the hot milk into the egg mixture, whisking constantly. Return the mixture to the pan and cook over medium heat, whisking constantly until the mixture thickens and comes to a boil again. Take care to scrape the sides and all over the bottom of the pan while whisking to prevent scorching or curdling.
On the lowest heat setting, continue stirring for 1 to 2 minutes—the mixture should become very smooth. Remove from the heat, discard the vanilla bean, transfer to a mixing bowl, and let cool to room temperature, stirring occasionally to prevent a skin from forming. Add the remaining yolk and the Sauternes (and the vanilla extract, if using) and stir until smooth.
Before serving, arrange the oven rack so it’s about 5 inches below the broiler element; heat the broiler.
In a small bowl, whip the cream until it forms soft peaks. With a spatula, gently fold the whipped cream into the pastry cream. Arrange the fruit evenly in a shallow ovenproof gratin dish, about 8x12 inches (or 6 individual dishes). Spoon the lightened pastry cream over the fruit.
Broil the gratin until golden brown, 5 to 7 minutes, turning the dish once or twice to avoid burnt spots. Serve hot or warm.
You can make the pastry cream (without the whipped cream) up to one day ahead, and keep covered in the refrigerator. Before whipping the heavy cream to fold in, bring the pastry cream back to room temperature and stir until smooth.