One 750-ml bottle brut Champagne or sparkling wine
1 dozen large egg yolks
2 cups heavy cream
In a saucepan, bring the water, sugar and vanilla bean and seeds to a boil. Reduce the heat to moderately high and cook, without stirring, to a light golden caramel, 10 minutes. Add the Champagne and simmer until reduced to 2 cups, 20 minutes.
In a large, heatproof bowl, lightly beat the egg yolks. Very gradually whisk in 1/4 cup of the hot Champagne caramel until blended. Slowly whisk in the remaining caramel. Set the mixture over a saucepan filled with 1 inch of simmering water. Cook over low heat, whisking constantly, until the custard thickens enough to form a ribbon when it falls from the whisk, about 10 minutes. Remove the vanilla bean. Transfer the custard to a bowl and let cool, then refrigerate until chilled.
Whip the cream just until firm. Fold into the Champagne custard and transfer to a serving bowl; serve cold.
The sabayon can be prepared through Step 2 up to 2 days ahead.