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Heat oven to 180C/160C fan/gas 4. Beat the sugar and butter until pale and creamy, then beat in the egg. Stir in the flour, coconut, crème fraîche and lemon zest.
Put most of the blueberries in a baking dish (about 18cm square-ish) and squeeze over the juice from half the lemon. Dollop on the cake mixture and scatter over the remaining blueberries and 2 tsp coconut. Bake for 35-40 mins until golden, risen and the sponge is cooked. Serve with a dollop more crème fraîche.