Warm milk, half the sugar, and one vanilla pod in a saucepan for 3-4 minutes until sugar melts.
Whisk egg yolks, flour, and remaining custard sugar into a smooth paste.
Stir warm milk into the egg paste and heat over low heat for 10-15 minutes until thickened.
Transfer custard to a bowl, cover with film, and set aside to cool.
Simmer rhubarb, wine, sugar, jam, and vanilla for 4-5 minutes until fruit softens.
Remove rhubarb to drain and simmer the remaining liquid for 2-3 minutes into a syrup.
Pulse gingernut biscuits in a food processor until they resemble large breadcrumbs.
Fold the whipped cream into the cooled custard.
Layer rhubarb, syrup, biscuit crumbs, and custard into cocktail glasses and garnish with mint.