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Heat oven to 180C/160C fan/gas 4. Grease and line two 20cm cake tins with baking parchment – if your cake tins are quite shallow, line the sides to a depth of at least 5cm. Put the flour, cocoa powder, baking powder, bicarbonate of soda, sugar and 1 tsp salt in a bowl and mix well. If there are any lumps in the sugar, squeeze these through your fingers to break them up.
Measure the buttermilk, coffee, oil and vanilla in a jug. Add the egg and whisk until smooth. Pour the wet ingredients into the dry and whisk until well combined. Pour the cake mixture evenly into the two tins, and bake for 25-30 mins until risen and a skewer inserted into the centre comes out clean. Cool in the tins for 10 mins, then turn out onto a wire rack, peel off the baking parchment and leave to cool. These sponges can be made up to three days ahead and will stay moist if wrapped in cling film, or you can wrap well and freeze for up to two months.
To make the mousse, put the gelatine leaves in a bowl of cold water, one at a time, so they don’t stick together. Set aside for 5 mins to soften. Scoop the pulp and seeds from the passion fruit into a sieve suspended over a bowl, and push through the juice – you should have about 100ml. Put 1 tbsp of the seeds in a small bowl and set aside. Save 2 small empty passion fruit halves and discard the rest.
Pour the coconut cream, passion fruit juice and sugar into a saucepan and bring to a gentle simmer. Remove from the heat and stir in the coconut rum liqueur. Add the gelatine to the hot liquid, a leaf at a time, squeezing out the water before you add them. Stir until the gelatine has dissolved. Fill each reserved passion fruit half with some of the mixture, place in a small dish and chill until needed. Leave the remaining mixture to cool to room temperature. Pour the double cream into a large bowl and whip until it’s holding its shape. Add half the passion fruit mixture, fold together, then add the remaining mixture and fold through again.
Line a deep 20cm cake tin with a double layer of cling film. Slice each sponge in half through the middle and drizzle each layer with 1 tbsp coconut rum liqueur. Place one of the layers into the tin, flat-side down, and spread with a third of the passion fruit mousse – don’t worry if a little trickles down the sides. Layer the remaining sponges and mousse on top, finishing with a layer of cake. Cover the top of the cake with cling film and chill for at least 4 hrs, or preferably overnight.
When the cake has chilled and the mousse is set, turn out onto a cake stand, so the base now becomes the top, and remove the cling film. Using a palette knife, scrape any passion fruit mousse off the edges of the cake, to give it a smooth surface. Place the cake back in the fridge while you prepare the ganache.
Put the chocolate and cream in a pan and gently heat, stirring now and then, to make a runny chocolate sauce. Leave to firm up at room temperature until it reaches a spreadable consistency.
Remove the cake from the fridge. Spread the ganache over the top and sides of the cake with a palette knife until completely covered. Decorate with a ring of toasted coconut shavings, if you like. Remove the filled passion fruit shells from the fridge and place these on top too, topped with the reserved passion fruit seeds. Chill until ready to serve, but bring out of the fridge 1 hr before slicing (don’t forget to serve the passion fruit jellies to your favourite guests too). Leftovers will keep in the fridge for 2 days.