How to Make It
Beat yolks in a heatproof bowl, preferably copper, until combined. Add sugar and salt, whisking constantly, until combined.
Pour wine into yolk mixture, and whisk until sugar is dissolved, about 30 seconds.
Place bowl over a saucepan of barely simmering water, and whisk mixture vigorously to incorporate air into mixture. Whisk until custard is warm, tripled in volume, and leaves a ribbon on surface as it falls from whisk, 8 to 9 minutes. Remove from heat. Place bowl in an ice bath, and whisk until custard reaches room temperature.
Stir mascarpone until smooth, then fold into zabaglione. Custard will keep in refrigerator up to 3 hours.