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Heat oven to 180C/160C fan/gas 4. Grease and line the base of a 23cm square cake tin with baking parchment. Butter the paper generously and sprinkle with the demerara sugar. Halve the plums and arrange in the base of the tin in 1 layer, cut-sides down.
For the cake, melt the butter, muscovado sugar and syrup in a large pan over a low heat, stirring until smooth. Cool for 10 mins, then stir in the eggs and milk. Sift in the flour, bicarbonate of soda and spices, then mix to a smooth batter.
Pour the batter into the tin, over the plums, and bake for 45-55 mins until firm to the touch. Cool in the tin for 10 mins, then turn out onto a wire rack and leave to cool. Will keep in the fridge, wrapped in baking parchment and foil, for up to 5 days.