Mix custard powder and sugar in a saucepan, add 4 tbsp of milk and mix the ingredients to form a smooth paste.
Split the vanilla bean in half lengthwise and scrape the seeds out with the blade of a knife.
Add the rest of the milk, cream and vanilla bean seeds and set over the heat.
Heat gently whisking the whole time to prevent lumps.
Custard will start to thicken after 5-7 minutes on the stove and will be as thick as gravy.
When it's desired consistency take it off the heat and stir in butter and rhubarb syrup.
Cool for 10 minutes.
Assemble the cake by filling each layer with a thin layer of rhubarb custard.
Decorate the top with reserved rhubarb pieces.
Chill for at least 2 hours before serving.