Mango purée subtly flavors this smooth-as-silk crème brûlée. Be sure to use the sweetest, ripest mangos you can find for the best mango flavor.
This dessert can be made ahead and refrigerated, covered, for up to a day before caramelizing the sugar topping. The caramelizing step should be done just before serving so that the burnt sugar is hard and crunchy.
A word about serving: Most people love the crispy caramelized topping, so select wide, shallow ramekins or gratin dishes to maximize the proportion of sugar crust to custard. Cooking time is shorter, too, if the custard is only about 1/2 to 1 inch deep.
Position a rack in the middle of the oven, and preheat to 325 degrees F.
Set 8 shallow (2/3-cup capacity) ramekins on a baking tray or in a shallow roasting pan and set aside. Place the vanilla beans, milk, and heavy cream in a small saucepan and bring to a simmer over medium heat. Once a simmer is attained, turn off the heat and let the mixture steep for 20 minutes to release the flavor of the vanilla. Remove the beans and scrape the seeds from the pods into the milk mixture. Wash the beans and reserve for another use.
Return the pan to the stove and add the sugar to the milk mixture. Reheat gently, stirring to dissolve the sugar.
In a large mixing bowl, whisk the egg and yolks until smooth. Add the mango purée and the warm milk mixture, blending well — but try to avoid creating froth or over-whisking.
Divide the mixture between the ramekins. Add enough boiling water to come halfway up the side of the ramekins (this step can be safer to do after you've transferred the baking pan to the oven). Cover the pan tightly with aluminum foil and bake for 35 to 45 minutes, or until the custard is just set but still wiggles slightly in the center. Cooking times will vary depending on the size of your ramekins.
Remove the tray from the oven and let the custards cool for 15 minutes. Remove the ramekins from the water bath and cool on a wire rack for 30 minutes more. Transfer the ramekins to the refrigerator and chill the custards completely, at least 3 hours. If refrigerating overnight, cover the custards with plastic wrap once they're chilled.
To serve, spread a thin layer of sugar over the top of one custard, smoothing it with the back of a spoon. Place the ramekin on a baking tray and caramelize the sugar with a butane torch, moving the tip constantly to avoid charring the sugar in any one area. Repeat with the other custards, and then serve within an hour, garnishing each with a slice of fresh mango, if desired.