Zabaglione is a cooked custard made of sweet wine, sugar, and egg yolks, whipped as it cooks over simmering water. We serve ours in champagne glasses, ladled over fresh seasonal fruit.
1 lb stone fruit (peaches, plums, etc.) or 1 lb berries
2 tsp Traditional (18 year) Balsamic Vinegar
1/3 Cup + 1 Tbsp granulated sugar
4 large egg yolks
1/3 Cup sweet Marsala wine
If using stone fruit: bring a pan of water to a simmer. Use a sharp paring knife to score the bottom of each fruit with a cross (X). Drop fruit into the simmering water for about 1 minute. Use a slotted spoon to remove fruit from hot water and immediately drop into a bowl of ice water. Peel fruit. Remove pit, and cut fruit into wedges.
Place prepared stone fruit or berries in a large bowl with 1 Tbsp sugar and balsamic vinegar. Lightly crush the fruit with a fork while stirring to coat fruit with sugar and balsamic. Allow fruit to macerate for at least 30 minutes at room temperature.
Meanwhile, combine egg yolks, Marsala wine, and the remaining sugar in a large metal bowl (or top pan of a double boiler) set over a saucepan (or bottom of a double-boiler) of 2 Cups barely-simmering water and beat with a handheld electric mixer or immersion blender at medium-high speed until mixture measures 140°F on an instant-read thermometer, about 7 minutes. Continue beating the mixture over the simmering water until zabaglione has tripled in volume and forms a thick ribbon when beaters are lifted, about 4 minutes more. Remove bowl from saucepan.
Divide fruit and its juice among 4 dessert bowls or glasses, and top with zabaglione. Zabaglione will begin to deflate as it comes to room temperature. Serve immediately.