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Heat the oven to 200C/180C fan/gas 6. Butter a deep 30 x 23cm rectangular tin. Beat the butter and sugar together in a stand mixer until pale, about 5 mins. Add the eggs, one at a time. Sift in the flour and baking powder, and add the lemon zest and milk. Fold in using a large metal spoon. Spoon into the tin. Bake for 35-40 mins until a skewer inserted into the centre comes out clean. Leave to cool for 10 mins in the tin, then turn out onto a wire rack to cool completely.
Transfer the cake to a board and cut in half through the equator. Put one half on a baking tray. Mix the caster sugar with the lemon juice, then brush this over the cut sides of the cake halves. For the buttercream, beat the butter in a stand mixer to loosen. Slowly add the icing sugar and a splash
of water and continue to beat until pale. Spread the jam over the base cake layer, then carefully spread a third of the buttercream on top. Sandwich with the other cake layer, then cut the cake into the shape of your desired number using a serrated knife, trying to waste as little of the cake as possible (offcuts can be eaten as a small treat). Spoon the remaining buttercream into a piping bag fitted with a star nozzle and pipe swirls on the cake. Decorate with your chosen toppings. Will keep chilled for up to three days.