Combine in the top of a double boiler or a heatproof bowl, off the heat:

Whisk vigorously until thick and pale yellow. Whisking constantly, gradually add:

Scrape the sides of the double boiler top clean with a silicone spatula, then set over very gently simmering water. Whisking constantly, heat to 160°F on an instant-read thermometer, by which point it will have increased several times in volume and become thick enough to mound very softly on a spoon. The cooking should take 5 to 10 minutes. If the zabaglione appears to be heating too quickly, periodically remove it from the water and whisk vigorously to cool. Serve the zabaglione warm or cool.

For a lighter zabaglione, set the top of the double boiler in ice water and gently whisk the sauce until cool to the touch. Beat in a medium bowl until stiff peaks form:

and fold into the cooled custard. Spoon into cups or stemmed glasses and serve immediately.