1. Grate the zest and put aside.
2. Pour the juice into a small saucepan and bring to a boil. 
3. Boil rapidly to reduce by half - approximately 10 minutes - then reduce to a simmer.
4. Beat the yolks and the zest with electric beater until pale.
5. Add the sugar to the juice and stir until dissolved.
6. Pour the simmering liquid onto the eggs while beating, until cool and very thick.  
7. Fold in the cream and freeze. Pour over a shot of vodka before serving.

CHEF'S TIP To get the maximum vitamin boost from citrus - squeeze, zest or slice just before using.