Preheat the oven to 200°C/fan 180°C/gas mark 6. Cut the rhubarb into 5cm lengths. Place in a shallow roasting tin so they sit in one layer. Add 50g of the sugar and turn the rhubarb to coat well.
Cover the tin with foil and bake for 15 minutes until the rhubarb is tender but still retains its shape. Leave to cool.
Meanwhile, mix together the remaining sugar, oil, egg and orange zest in a bowl, but be careful not to over beat. Stir through the flour and cream, then gently fold in the rhubarb. Divide the mixture between a 12-hole muffin tin lined with paper cases. Bake for 20-25 minutes until risen and golden.
Cool for 10 minutes, then cut a small deep cross in the top of each muffin and spoon on 1 tsp custard. Leave to settle before adding another 1 tsp custard on top. Dust with icing sugar and serve warm.