Heat oven to 180C/160C fan/gas 4. Butter and line two 20cm cake tins. Beat butter and sugar until pale and fluffy. Gradually add eggs, beating well. Fold in flour, baking powder, and cocoa. If stiff, add milk.
Spoon into tins and bake for 20-25 mins until a skewer comes out clean. Turn out onto a wire rack, removing parchment. Heat lime juice and sugar, stirring to dissolve. While warm, pierce cakes with a skewer and drizzle with syrup. Cool completely.
For candied zest, pare lime zest into broad strips, removing white pith. Slice into matchsticks. Boil in water for 1 min, drain, rinse, and dry. Juice limes, add water to make 225ml, and heat with sugar until dissolved. Simmer zest in syrup for about 30 mins until nearly evaporated. Remove with a slotted spoon and dry on parchment.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. For the icing, gently heat the cream without boiling, then remove from heat. Add chocolate, stirring until melted. Warm marmalade with 1 tbsp cream until dissolved, then mix into the chocolate. Let cool slightly but not until firm. Pour over one sponge layer, allowing it to drip down the sides. Let set. Spread buttercream on one layer, then top with the chocolate-coated layer. Decorate with candied lime zest.