For the muffin mix
175g caster sugar
175g rhubarb, halved lengthways then diced
2 tbsp sunflower oil
1 egg
1 tsp vanilla extract
125ml buttermilk
200g plain flour
1 tsp baking powder
1 tsp bicarbonate of soda
For the crumble topping
50g light muscovado sugar
50g plain flour
25g porridge oats
1 tsp ground cinnamon
50g butter
Method
Preheat the oven to 220C / 200C fan /gas mark 7. Fill a 12-hole muffin tray with paper muffin cases.
Mix the sugar and rhubarb together and set on one side.
To make the crumble topping mix together the muscovado sugar with the flour, oats and cinnamon. Rub the butter into the mixture until it comes together into crumbly clumps.
Beat the oil and the egg lightly together, then stir in the vanilla and buttermilk. Pour the mixture over the sugar-coated rhubarb. Add the flour, baking powder and bicarbonate of soda and mix well.
Spoon the mixture into the cases, then cover each one with a thick layer of the crumble mixture. Bake for 15-18 mins until golden and a skewer poked into the centre of a muffin comes out clean. Leave to cool on a wire rack.