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Heat the oven to 200C/180C fan/gas 6. Butter a 25cm square or 20 x 30cm rectangular cake tin or baking dish.
Beat the butter, sugar, flour, eggs and vanilla extract together until smooth. Spread the jam over the base of the tin, and dollop over the cake batter. Spread over evenly. Bake for 20-25 mins until the sponge is light golden and risen, and a skewer inserted into the middle comes back clean. Serve warm with custard, cream or ice cream.