Have ready an instant-read thermometer in a cup of hot water. In a metal bowl with a whisk or a hand-held electric mixer beat together all ingredients until combined well. Set bowl over a saucepan of barely simmering water and beat mixture until tripled in volume and thermometer registers 140 degrees, about 5 minutes. To ensure that eggs are cooked, beat mixture 3 minutes more. Serve zabaglione immediately.