1.Preheat oven to moderate, 180°C (160°C fan-forced). Lightly spray a 12-hole muffin pan with oil.
2.To make custard: In a saucepan, combine custard powder, sugar and a little milk to form a smooth paste. Blend in remaining milk until well combined. Stir over low heat until mixture boils and thickens. Reduce heat. Simmer 3 minutes, stirring. Remove from heat. Cover surface of custard with plastic wrap to prevent a skin forming. Let cool.
3.Sift flour into a large bowl. Stir in caster sugar. In a jug, combine milk, butter and egg.
4.Make a well in the centre of the dry ingredients. Add rhubarb and milk mixture all at once. Mix lightly until just combined. Do not over-mix.
5.Half-fill each muffin-pan recess with muffin batter, then spoon custard evenly on top. Spoon remaining batter on top. Sprinkle with demerara sugar.
6.Bake 20-25 minutes, until cooked when tested. Serve warm.
Prepared custard could be used in this recipe.Note