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Heat oven to 180C/160C fan/gas 4. Butter and line the base of 2 x 20cm cake tins with baking parchment. Beat the butter and light brown sugar together until pale and fluffy. Add the eggs gradually, beating well between each addition. Fold in the flour, baking powder and cocoa powder. If the mixture is very stiff, stir in the milk.
Spoon into the cake tins and bake for 20-25 mins or until a skewer inserted into the centre comes out clean. Turn the cakes out onto a wire rack and peel off the parchment. Gently heat the lime juice and caster sugar together, stirring to help the sugar dissolve. While the cakes and the syrup are still warm, pierce the cakes all over using a skewer and spoon over the syrup. Leave to cool completely.
Next, make the candied lime zest. Pare the zest from the limes, cutting it into broad strips. Cut away the white pith and discard, then slice the zest into matchstick-sized strips. Put them in a pan and cover with water, then bring to the boil. Cook for 1 min, then drain and rinse. Dry and set aside. Juice the limes and add enough water to make 225ml. Heat gently with the sugar until the sugar has melted. Add the strips of zest and simmer until the liquid has almost evaporated and turned syrupy – about 30 mins. Scoop the strips of zest out with a slotted spoon, then place them on parchment to dry.
To make the buttercream, beat the butter and icing sugar together using an electric mixer or by hand (start off gently or the icing sugar will fly everywhere), until pale and light, but don’t beat for so long that it gets too fluffy. Add the lime zest and beat again until smooth. Chill in the fridge until the buttercream is firm, but spreadable.
To make the icing, heat the cream gently (don't let it boil), then take it off the heat. Add the chocolate and let it melt, stirring to help it along. Put the marmalade in a small pan with the extra 1 tbsp cream and heat slowly to dissolve. Add to the chocolate and cream and stir until everything is blended. Leave to cool a little, but not so long that it becomes firm. Pour over one of the layers of sponge and leave to cool and set – it will run down the sides and drip off.
Spread the buttercream over the other layer, then put the chocolate coated layer on top. Decorate with the candied lime zest.