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Heat oven to 180C/fan 160C/gas 4. Butter and line the base of a round 20cm cake tin. Halve and stone 4 plums, set aside for later, then roughly chop the remaining plums.
Put the sugar, butter, flour, ground hazelnuts, eggs and baking powder into a large bowl and beat with a wooden spoon or electric hand mixer for 1-2 mins, until smooth and light. Stir in the chopped plums and chocolate, then tip into the prepared cake tin and smooth the top.
Arrange the halved plums over the top of the mixture, pressing them down lightly, then scatter over hazelnuts. Bake for 40-50 mins until the top is golden and the cake feels firm to the touch. Cool in the tin for 10 mins, then turn out, remove the paper and cool on a wire rack. Heat the jelly, then brush over the top of the cake before serving.