Bring the Riesling with the grape juice and the vanilla pod to the boil once. Beat the egg yolks with the sugar and butter until frothy and then slowly add the hot (not boiling) wine, stirring constantly. Beat the mixture thoroughly with a whisk and let it cool in a water bath. Add the lime zest and let the mixture freeze in the ice cream machine, approx. 1.5 hours.
In the meantime, the grape compote can be prepared: Bring the grape juice, port wine, sugar and star anise to the boil and reduce while simmering. Meanwhile, halve and core the grapes. Mix the cornstarch with a dash of water to avoid lumps, add to the wine brew and bring to the boil together. Remove the star anise, pull the pan off the stove, add the grapes and let it steep for a few minutes. If the compote is not served immediately, it can be warmed up again just before serving.
Serve the Riesling ice cream with the grape compote - which is not so easy, because the ice cream then naturally melts quickly.