Preheat the oven to 200°C / 180°C (fan) / 425°F / Gas mark 7. Line 12-hole (80mm) muffin pan with paper cases.
For the custard:
Combine custard powder and sugar in a saucepan.
Gradually stir in the milk over a medium heat until the custard boils and thickens. Stir in the vanilla and leave to cool.
For the muffins:
Sift the flours and caster sugar into a large bowl. Combine butter, milk and egg and stir into dry ingredients. Do not over mix; mixture should be lumpy. Stir in ¾ of the rhubarb.
Divide half the mixture among the paper cases then top with custard.
Divide remaining mixture over the custard, ensuring cases are no more than ¾ full.
Sprinkle with remaining rhubarb and demerara sugar.
Bake for 30 minutes on the middle shelf.
Leave to cool on wire rack. Lightly dust with sifted icing sugar.