Warm the single cream in a saucepan but
do not let it boil. Whisk the eggs, egg yolks and all but 15 ml/1 tbsp
of the sugar together until thick and pale. Whisk in the warm cream
and the vanilla. Place the bowl over a pan of gently simmering water
and continue whisking until the mixture thickens and will coat the
back of a spoon. To test, lift the spoon out of the mixture and draw a
finger through the mixture on the spoon: if it leaves a clear line,
the custard is ready. Stand the bowl in cold water to cool quickly.
Sprinkle the surface with the reserved caster sugar to prevent a skin
forming and leave until cold.
Whip the double cream until softly peaking, then whisk into the cold
custard. Turn into a rigid container, cover and freeze for 1 hour.
Whisk thoroughly with a fork to break up the ice crystals. Repeat the
freezing and whisking twice more, then freeze until firm.