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Heat oven to 200C/180C fan/gas 6. Use 1 tsp oil to lightly oil a 12-hole muffin tin (or use paper cases). Mix both flours with the baking powder and lemon zest. Reserve 1 tbsp caster sugar, then stir the rest into the flour with the muscovado.
Mash the banana well. In another bowl beat the egg, then stir in the banana, buttermilk and oil. Using a large metal spoon, very lightly stir into the flour mix, just to combine. Over-mixing will make the muffins tough. Toss in the blueberries and give just a few turns of the spoon to carefully stir them in without crushing.
Spoon the mixture into the tin – each hole should be very full. Bake for 20-25 mins until risen and golden.
Mix the reserved caster sugar with the lemon juice. When the muffins are done, remove from the oven, then brush with the sugar and lemon mixture while they are still hot. Gently loosen the edges of each muffin with a knife, then leave in the tin for 15 mins to cool a little as they’re very delicate while hot. Remove to a wire rack. Best eaten the day of making, but will keep for up to 2 days.