Beat the egg yolks in a heatproof bowl with the sugar and vanilla sugar, using a hand whisk, until frothy.
Bring the milk to the boil then pour through a sieve onto the egg mixture, whisking constantly.
Set the bowl over a pan of warm simmering water and whisk until thick and creamy.
The mixture is ready when a finger drawn through the mixture on the spoon leaves a clear gap.
Set the bowl over iced water and whisk until cold.
Whip the cream until stiff and fold it into the ice cream mixture.
Rise a ring cake tin with cold water, empty and pour in the ice cream mixture. Freeze for at least 4 hours.
For the chocolate sauce, bring the cream to the boil. Chop the chocolate and stir into the hot cream until melted.
Dissolve the coffee in 4 tbsp hot water and fold into the chocolate sauce. Allow to cool.
Briefly dip the cake tin in hot water and turn out the ice cream.
Place a ring (slightly larger than the diameter of the cake tin) on a plate and fill with double cream. Pour some chocolate sauce around.
Remove the ring and draw a decorative pattern with a cocktail stick or skewer. Place the ice cream cake in the middle and pour over the remaining chocolate sauce. Serve immediately.