Warning in advance: this cake is sweet. Super sweet, and so not for the faint hearted. You can use dark chocolate in place of the white which will lessen the sweetness, if that is your preference, and swap the mango for berries or figs to really make this cake your own.
Preheat the oven to 180C/350F/Gas 4 (160C fan). Grease two 18cm/7in loose bottomed, square sandwich tins with a little oil and then line with baking parchment. Sit on a baking sheet and set aside.
Now make the sponge. It is easiest to use a food mixer set with a beater attachment for this, but you can also do it by hand. Cream together the butter and sugar until nicely combined. Add the eggs, one at a time, beating well between each addition. Then fold in the flour until the mixture is uniform and smooth. Divide the mixture evenly between the tins, spreading it out smoothly with the back of a spoon. Bake for 18-20 minutes.
Meanwhile, prepare the chocolate shards. Cut out a 28x33cm/11x13in piece of baking parchment and place it on a baking sheet. Melt the chocolate either in a bowl sitting on top of a pan of simmering water (do not allow the bowl to touch the water) or in 30 second blasts in the microwave, stirring between each go. Once melted, stir the chocolate until smooth and then pour it onto the centre of the prepared baking parchment. Pop a tiny dot of melted chocolate under each corner of the paper to stick it to the baking sheet and prevent it from moving about. Using a palette knife, spread the chocolate all over the paper right out to the edges in a smooth, even layer. Pop in the fridge for 30 minutes, or until set firm.
Meanwhile, prepare the icing. (Again this can be done in a food mixer set with the beater attachment or by hand.) Give the butter a quick beat to loosen it and then add the icing sugar and lemon and lime zests. Beat together until fully combined, smooth and creamy in texture. Set aside until ready to use.
To check the cakes are cooked, insert a skewer into the centre of the cakes – if they are cooked it will come out clean. Once baked, remove from the oven and set aside to cool completely.
Remove the set chocolate from the fridge once firm. Dip a long, sharp knife into really hot water and wipe dry. Run the knife under the paper to release it from the tray and then cut the chocolate into twelve 11x7cm/4¼x2¾in rectangles. Carefully peel each one from the backing paper and lay on a clean board. Trim any ragged edges to neaten the shards up. Then make a cut on the diagonal about 3cm/1¼in down one long side for a nice decorative finish. If the shards feel like they are getting too soft at any point return them to the fridge briefly until hardened. Likewise, if they are shattering when being cut, it means they are too hard and brittle and so leave them to warm to room temperature before continuing. Return the prepared shards to the fridge until needed.
Once cooled, remove the cakes from the tins and peel the paper away. Spread a small dot of buttercream icing onto the centre of a serving plate or cake stand. Sit one cake half on top to stick and then spread the top of this evenly with a third (about 200g/7oz) of the buttercream icing. Sit the remaining cake on top to sandwich the filling and then spread the remaining buttercream icing all over the top and sides of the cake evenly. It doesn’t have to look perfect as the whole cake will soon be covered with chocolate shards and fruit.
To decorate, stick a shard, pointed side up and nicest side out, against the side of the cake, starting at one edge. Then add another, overlapping it slightly over the first. Then add a third in the same way to complete one side. Repeat to use up all the shards and cover all sides of the cake.
Scatter the mango pieces in a single layer all over the top of the cake. Sprinkle over the coconut and lime zest (if using) and it is ready to serve. If not serving straight away, this is best kept in the fridge to prevent the shards from melting. It can be prepared up to a day in advance.
