For the custard, warm the milk, half of the sugar and the vanilla pod in a saucepan over a low to medium heat for 3-4 minutes, or until the sugar has melted.
In a mixing bowl, whisk the egg yolks, flour and the remaining sugar until the mixture resembles a smooth paste. Pour in the warm milk mixture and stir until well combined, then transfer the mixture to a separate saucepan and heat over a very low heat for 10-15 minutes, or until the mixture is thick enough to coat the back of a spoon. Transfer to a bowl, cover with cling film and set aside to cool.
For the gingernut fool, place the rhubarb batons, wine, sugar, jam and vanilla pod into a saucepan and simmer over a medium heat for 4-5 minutes, or until the rhubarb begins to soften. Scoop out the rhubarb and transfer to a baking tray to drain.
Continue to simmer the remaining sugar mixture for 2-3 minutes, or until the volume of liquid has reduced and has the consistency of syrup. Set aside to cool.
Blend the gingernut biscuits in a food processor until they resemble large breadcrumbs.
When the custard has cooled, fold the whipped cream into it.
To serve, spoon some of the rhubarb and syrup into the bottom of each cocktail glass. Sprinkle in some of the biscuit crumbs, then add the custard. Repeat until the glasses are almost full, then top with the remaining rhubarb pieces and drizzle with the syrup. Garnish with a sprinkling of the gingernut crumb and a mint leaf, if desired.