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Heat the oven to 180C/160C fan/gas 4 and line a 12-hole muffin tin with paper cases. Beat 100g of the butter and the caster sugar together using an electric whisk until pale and fluffy. Add the eggs, beat for 1 min, then mix in the yogurt, vanilla and milk. Combine 250g of the flour with the baking powder and bicarb in a bowl with ¼ tsp salt, then stir this into the wet ingredients.
Spoon 2 tbsp of the batter into each paper case. Gently push two raspberries each into four of the cases, two or three blackberries into another four and a large spoonful of chopped peaches into the final four. Divide the rest of the batter between the cases, spooning it over the fruit, then top with more berries or peaches, to match the filling.
Mix the remaining 50g flour and 25g butter together with your fingers until the mixture clumps together in small chunks, then stir in the demerara sugar. Sprinkle a little of the crumble mixture over each muffin, then bake for 22-25 mins until risen, golden and a skewer inserted into the centre comes out clean.
Leave to cool in the tin for 10 mins, then carefully lift out onto a wire rack to cool completely. Will keep for three days in an airtight container.