Stir egg yolk and 100 g sugar in a hot water bath for about 5 minutes until creamy. Leave to cool down a little.
Line a box mould (approx. 20 cm long; approx. 1 l capacity) with foil or provide an empty clean plastic ice cream container. Chop chocolate. Whip cream and vanillin sugar until stiff.
First add 100 ml egg liqueur, then chocolate chips and finally cream to the egg mixture. Pour the mixture into the mould and smooth it down. Cover with foil and freeze for at least 6 hours, preferably overnight.
Drain the cherries while collecting the juice. Stir 2 tablespoons of juice and starch until smooth. Boil up the rest of the juice and 1-2 tbsp. sugar. Bind with the mixed starch and simmer for about 1 minute. Fold in the cherries.
Let it cool down.
Turn the parfait out of the mould 15-20 minutes before serving to make it easier to cut. Then cut it into pieces, dipping the knife in hot water from time to time. Serve with the cherry sauce and sprinkle with the remaining advocaat.