This simplified version of Peter’s gluten-free Graduation Cake is no less impressive. The sponge is made using coconut yoghurt to give it a rich flavour, while a fresh mango topping and homemade mango curd provide tropical zing.
Heat the oven to 190°C/170°C fan/Gas 5.
Beat the butter and caster sugar in a stand mixer fitted with the beater attachment, on medium speed for about 3 minutes, until pale and light.
Add the egg whites, coconut milk, coconut yoghurt, vanilla extract, flour, baking powder and salt and mix gently until just combined. Then, beat until smooth.
Fold through the desiccated coconut and white chocolate and divide the mixture equally between the prepared tins. Bake for about 25 minutes, until the cakes are risen, golden, coming away from the sides of the tins, and a skewer inserted into the centers comes out clean. Leave to cool in the tins for 2–3 minutes, then turn out onto a wire rack to cool completely.
Meanwhile, prepare mango curd. Whisk together the egg yolks, caster sugar, mango pulp and lime juice. Pour the mixture into a small, heavy-based pan and set over a low heat. Stir continuously until the eggs have cooked and the curd thickens and coats the back of a spoon.
Remove from the heat and whisk in the butter a few cubes at a time. Strain the curd into a clean bowl, cover the surface with cling film, cool to room temperature, then chill until firm (at least 3 hours) or until needed.
To make the buttercream, melt the white chocolate in a bowl set over a pan of simmering water, stir until smooth and leave to cool. Put the butter and coconut cream in the bowl of a stand mixer fitted with the beater attachment, and beat on medium speed until pale and light.
Gradually add the icing sugar, mixing well between each addition, until the buttercream is fully combined and the mixture is light and fluffy. Add the cooled white chocolate and the vanilla and mix again until smooth. Cover and chill for 10 minutes to firm up slightly, then scoop 4–5 tablespoons of the buttercream into the piping bag fitted with the medium plain nozzle.
Pour the cream into a bowl and, using an electric hand whisk, whisk it until it holds stiff peaks.
To assemble the cake, lay one sponge layer on your serving plate and pipe a ring of buttercream around the edge. Spread half the mango curd inside the buttercream border and top with half the whipped cream. Top with the second sponge layer and repeat the fillings, then top with the final sponge. Coat the sides and top of the cake with a thin crumb coat of buttercream and chill for 30 minutes.
Meanwhile prepare the chocolate collar. Melt the white chocolate in bowl set over a pan of gently simmering water and stir until smooth. Remove the bowl from the heat and leave the chocolate to cool slightly. Lay the acetate strip on the work surface on top of a sheet of baking paper.
Spoon 1½ tablespoons of melted chocolate into a small bowl, add a tiny amount of yellow food-colouring paste and mix until the chocolate is bright yellow. Spoon 1½ tablespoons of melted chocolate into another small bowl, add a tiny amount of red food-colouring paste and mix until the chocolate is bright red. Pour each coloured chocolate into a piping bag fitted with a fine writing nozzle and pipe swirls of each all over the acetate strip. Chill for 10–15 minutes, until set firm.
Using a palette knife, spread the remaining melted white chocolate over the chilled swirls in an even layer. Leave at room temperature until cold and the chocolate has nearly set.
Cover the cake in the remaining buttercream and carefully wrap the chocolate collar around the outside, leaving the acetate in place. Place the cake in the fridge for about 10 minutes, until the collar has set firm, then carefully peel away the acetate.
Mix the diced mango with the lime zest in a small bowl and arrange this over the top of the cake. Scatter with toasted coconut chips to finish.