Place egg yolks and sugar in a large, round-bottomed stainless steel bowl and wisk for about 10 minutes, until the mixture triples in volume, froths up and becomes pale.
Pour in the wine in a slow, steady flow.
In the meantime, half-fill a pot with water, bring the water to a simmer and reduce the heat to low. Set the bowl containing the custard over the water in a bain-marie and make sure that the water does never boil, otherwise the custard will curdle.
Whisk the custard gently by hand while heating it; when it reaches the desired consistency, take the bowl of custard out of the pot.
Slightly thickened, the custard can be used as a sauce, while longer cooking will thicken the custard further, giving it the texture of a mousse.
Serve the custard while still warm or set it aside at room temperature for 15 minutes if you want it cold.
Ladle the zabaglione into individual glasses. Serve with berries or cookies such as traditional langues de chat.
Zabaione can be used instead of pastry cream to fill pies, tarts and éclairs.