DirectionsIn a bowl whisk together the yolks and the sugar and add the milk, scalded with the zest, in a slow stream, whisking.
In a heavy saucepan combine the yolk mixture and the heavy cream and cook the custard over moderately low heat, stirring constantly with a wooden spoon, until it registers 175F.
on a candy thermometer.
Pour the custard into a metal bowl and whisk in the vanilla and the sour cream.
In a small saucepan sprinkle the gelatin over 1/4 cup cold water, let it soften for 1 minute, and heat the mixture over moderate heat, stirring, until the gelatin is dissolved.
Stir the gelatin mixture into the custard, set the bowl in a larger bowl of ice and cold water, and chill the custard, stirring occasionally, until it is the consistency of raw egg white.
Line the bottom of a 1 1/2-quart glass bowl with some of the fruitcake slices, pour in the custard, and push the remaining fruitcake slices into the custard in decorative patterns.
Chill the trifle, covered, for at least 2 hours or overnight.