To prepare the ice cream, pour the milk into a saucepan and slowly heat it until it reaches boiling point. Do not allow it to boil.
Meanwhile, whisk the egg yolks and caster sugar together until light and creamy.
Whisking continuously, pour the hot milk over the egg mixture and return to the pan.
Heat gently, stirring constantly, until the mixture forms a film on the back of a wooden spoon. You'll be able to drag your finger over the back of the spoon and it will leave a line. Do not let it boil or it will separate.
Remove the custard mixture from the heat and stir in the broken pieces of white chocolate. Stir until melted. Cover in cling film and leave to cool to room temperature.
Add the cream and vanilla seeds and transfer the mixture to the fridge to cool completely for a few hours.
Meanwhile, to prepare the sorbet, coarsely chop the whole clementines and place in a saucepan along with the caster sugar and water. Bring to the boil and simmer, with the lid on, for 10 minutes.
Puree with a hand blender and leave to cool before adding the lemon juice and Goldschlager.
Transfer the mixture to a freezerproof container and freeze for a few hours.
When the ice cream mixture is cold, churn in an ice cream machine.
Line a 2 litre freezerproof bowl with clingfilm and spoon the ice cream into it. Cover the back of a 0.5 litre freezerproof bowl with cling film and press down into the centre of the ice cream to make a well. Place in the freezer for a few hours.
When the ice cream is hard, remove the bowl in the centre.
Transfer the frozen sorbet to a food processor and blend until it is light and creamy. Spoon the sorbet into the hole in the ice cream so that it is level. Return the ice cream to the freezer and leave over night.
To serve, tip the bowl upside down over a serving dish, remove the cling film and dust with icing sugar, if desired. Serve immediately.