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Heat oven to 220C/200C fan/gas 7. Line a 12-hole muffin tin with paper muffin cases. Stir the sugar and rhubarb together and set aside while you make the crumble topping. Mix together the muscovado sugar with the flour, oats and cinnamon, then rub in the butter until clumpy with your fingertips.
Stir the oil and egg, vanilla and buttermilk into the sugary rhubarb (don’t worry if it’s a little bit juicy). Now, add the flour, baking powder and bicarbonate of soda and stir well.
Quickly spoon into the cases, then scatter each with a thick layer of the crumble mixture. Bake for 15-18 mins until golden and a cocktail stick poked into the centre of a muffin comes out clean. Cool on a wire rack.