In a medium pot, whisk together the egg yolks and sugar; whisk in the cream. Put the pot over medium low heat and cook, whisking often, until the mixture bubbles and thickens; whisk in the mascarpone until it melts and the mixture is smooth. Whisk in the vanilla. Remove from heat and cool completely, then refrigerate until cold. Freeze according to the directions of your ice cream maker. Makes about 1 pint.