Makes 12 Muffins
1. Preheat the oven to 200°C / 180°C (fan) / 425°F / Gas mark 7.
2. Line 12-hole (80mm) muffin pan with paper cases.
3. 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 vanilla and leave to cool.
4. For the muffins: Sift flours and caster sugar into a large bowl.
5. Combine butter, milk and egg and stir into dry ingredients. Do not over mix; mixture should be lumpy.
6. Stir in ¾ of the rhubarb.
7. 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.
8. Sprinkle with remaining rhubarb and demerara sugar.
9. Bake for 30 minutes on the middle shelf.
10. Leave to cool on wire rack. Lightly dust with sifted icing sugar.