Preparation In a large metal bowl with an electric mixer beat together yolks and sugar until pale yellow and thick. In a metal bowl set over a pan of simmering water heat 2 cups heavy cream until warm. Add warm cream to yolk mixture in a slow stream, whisking constantly. Set bowl with custard over simmering water and cook, stirring constantly with a wooden spoon, until a thermometer registers 170°F. Remove bowl from heat and stir in remaining cup cream. Chill custard until cold, about 1 hour. Add Grape-Nuts and vanilla to custard and freeze in an ice-cream maker. Transfer ice cream to an airtight container and put in freezer to harden. Ice cream may be made 1 week ahead.
Preparation In a large metal bowl with an electric mixer beat together yolks and sugar until pale yellow and thick. In a metal bowl set over a pan of simmering water heat 2 cups heavy cream until warm. Add warm cream to yolk mixture in a slow stream, whisking constantly. Set bowl with custard over simmering water and cook, stirring constantly with a wooden spoon, until a thermometer registers 170°F. Remove bowl from heat and stir in remaining cup cream. Chill custard until cold, about 1 hour. Add Grape-Nuts and vanilla to custard and freeze in an ice-cream maker. Transfer ice cream to an airtight container and put in freezer to harden. Ice cream may be made 1 week ahead.