Four lime and four coconut sponge layers are sandwiched with rum buttercream to create this piña colada-flavoured celebration cake. If you can’t find candy melts, use white chocolate for the decorations.
Heat the oven to 180°C/160°C fan/350°F/Gas 4. For the coconut sponge, beat the flour, butter, sugar, eggs desiccated coconut and baking powder in a stand mixer fitted with the beater, on low speed for about 60 seconds, or until the batter comes together. Add the coconut cream, coconut essence and milk and briefly mix again until smooth. Divide the mixture proportionately between the two 20cm and two 15cm cake tins and bake for 23 minutes for the smaller cakes and 25 minutes for larger sponges, until a skewer inserted into the centres comes out clean. Allow to cool in the tins for 5 minutes, then turn out onto a wire rack to cool completely. When cool, trim the tops flat, if necessary.
For the lime sponges, beat together all the ingredients except the food colouring in a standing mixer fitted with the beater, as before, adding a little green food colouring at the end to give a pale shade. Divide between the cake tins and bake, cool and trim, as before.
Meanwhile, make the curd. Place the pineapple juice, lime zest and juice and sugar in a medium bowl and place over a pan of gently simmering water. Stir until the sugar has dissolved, add the butter and stir until the butter melts. Take the pan off the heat, beat the eggs and egg yolks together in a bowl, then whisk the mixture into the warm butter mixture. Return the pan to the heat, keep the heat low and stir gently with a wooden spoon for about 15 minutes, until the mixture thickens enough to coat the back of a spoon. Set aside.
For the Swiss meringue buttercream, place the egg whites and sugar into the clean bowl of a stand mixer and place over a pan of gently simmering water. Gently swirl with a balloon whisk, until all the sugar has dissolved (it should reach 71°C/160°F on a sugar thermometer and should feel smooth between your fingertips). Transfer the bowl back onto the mixer (fitted with the whisk) and whisk on medium–high speed until you have a thick meringue and the bowl feels completely cool to the touch. With the whisk still running on medium speed, gradually add the butter until the meringue is thick and creamy. Place 110g of the buttercream in a small bowl and colour with the yellow food-colouring paste. Cover both bowls with cling film and set aside.
For the rum buttercream, beat the butter in a stand mixer until smooth, then gradually add the icing sugar and rum and beat again until smooth. Cover with cling film and set aside.
Place a 20cm coconut cake on the 25cm board, securing it with a little rum buttercream. Spread a layer of rum buttercream on top of the 20cm cake, followed by a generous amount of the pineapple lime curd. Top with a 20cm lime cake. Again, spread with rum buttercream, then curd. Repeat for the remaining 20cm cakes to give 4 layers, placing the last cake bottom-side upwards so give a flat top. Spread a little of the Swiss meringue buttercream thinly all over the top and sides of the cake to create a crumb coat. Chill the layered cake for 15 minutes.
Next, place a 15cm coconut cake on the 15cm board, securing it with a little rum buttercream. Spread a layer of rum buttercream on top of the 15cm cake, followed by a generous amount of the pineapple lime curd. Top with a 15cm lime cake. Again, spread with rum buttercream, then curd. Repeat for the remaining 15cm cakes to give 4 layers, placing the last cake bottom-side upwards so give a flat top. Spread Swiss meringue buttercream thinly all over the top and sides of the cake to create a crumb coat. Set the remaining buttercream aside for later. Chill the layered cake for 15 minutes.
While the cakes are in the fridge, prepare the candy-melt shards. Lay out 2 large sheets of baking paper on a work surface. Melt the white candy melts according to the packet instructions. Once melted, use a teaspoon and drop 10–12 blobs of the mixture onto a sheet of baking paper. Using a palette knife, drag through the candy melts to make each one about 17cm long. Repeat with the green candy melts to make 3–5 green shards and again with the yellow candy melts to make 10–12 yellow shards.
Spread Swiss meringue buttercream around the base of the large cake, and all over the sides to give a thick coat, spreading evenly with an icing scraper (do this on a turntable, if you have one). Give the 15cm-cake stack a coating of Swiss buttercream in the same way. Stack the 15cm cakes on top of the 20cm cakes.
To decorate, sprinkle the coconut flakes around the bottom of both cake tiers. Arrange the yellow and white candy-melt shards around the top tier cake. Place 3–5 green shards on the top of the cake and decorate with the whole cherries, and the pineapple slices and coconut chunks.