Mix rhubarb with golden caster sugar. Mix plain flour with baking powder. Beat eggs with milk and melted butter. Heat the oven to 200C/fan 180C/gas 6, line a 12-hole muffin tin with paper cases. Bake the rhubarb for about 10 minutes until just tender. Drain really well and cool on kitchen towel.
Stir the wet ingredients into the dry ones along with the rhubarb (don’t overmix, it should be a bit lumpy). Divide between the muffin cases, sprinkle the tops with demerara sugar and bake for 25-30 minutes until risen and golden.