In a small saucepan bring the cream, ginger, and lemon zest just to a boil. Remove pan from heat and let mixture stand covered for 15 minutes.

 

In a bowl with an electric mixer, beat yolks with sugar until mixture is thick and pale. Whisk the yolk into the cream and heat over moderately low heat, whisking, until a candy thermometer registers 160 degrees F. Remove the pan from the heat.

 

Strain sauce through a sieve. Discard the lemon zest and the ginger. Serve over ice cream.