I first made this dessert when I was 15 for a dinner party with family friends. It was one of the first bakes I made that really wowed the table and has become a classic to bring out any time we want to feel a little fancy or show off a bit!
Heat the oven to 200°C/180°C fan/400°F/Gas 6. Make the sponge. Whisk the eggs and sugar in a stand mixer fitted with the whisk, on high speed for about 8 minutes, until thick, fluffy and mousse-like, and the mixture leaves a ribbon trail when you lift the whisk.
Sift the flour and cocoa powder into the bowl and fold in with a large metal spoon. Spread out the mixture in the lined Swiss roll tin, right into the corners. Bake for 10-12 minutes, until the sponge is risen and springs back when pressed. Cool in the tin for 10 minutes, then turn out onto baking paper and remove the backing. Cool completely.
Make the mousse. Soak the gelatine leaves in 50ml water. Place the pomegranate juice and defrosted or fresh berries in a liquidiser or food processor and blend until smooth, then pass the mixture through a sieve into a jug (discard the contents of the sieve). Pour 225ml of the juice into a pan and heat until hot, but not boiling.
Meanwhile, using a balloon whisk, whisk the egg yolks with 85g of the sugar in a bowl until pale. Gradually whisk in the hot berry juice, then return the mixture to the pan and cook over a low-medium heat, stirring, until the custard thickens and lightly coats the back of a spoon. Do not let it boil.
Take the custard off the heat. Squeeze out any liquid from the gelatine and stir the leaves into the custard until melted. Pass through a fine sieve into a clean bowl, cover the surface with cling film to prevent a skin forming and leave at room temperature to cool.
Place the egg whites and 30g of the sugar in the cleaned bowl of the stand mixer fitted with the whisk. Leave the mixture ready to go while you make a syrup for the mousse.
Gently heat the remaining 110g caster sugar in a pan with 35ml water until the sugar dissolves. Bring to the boil, without stirring, until it reaches 112°C on a sugar thermometer. When the temperature reaches 107°C, start whisking the egg whites and sugar in the stand mixer on high speed until they form stiff peaks. Then, once the sugar syrup reaches 112°C, slowly pour the hot syrup into the egg whites, whisking on high speed until it cools to a thick, glossy meringue.
In a separate bowl, whisk the cream to soft peaks then fold into the cooled meringue, then fold in the cooled berry custard.
To assemble, using the base of the springform tin as a template, cut out a circle of the sponge. Place the sponge inside the lined cake tin, then place the tin onto a serving plate. Pour the mousse into the tin, then chill for at least 4 hours, until set.
Make the jelly topping. Soak the gelatine leaves in a little cold water. Heat the sugar and 100ml water in a pan over a low-medium heat and bring to a simmer, stirring until the sugar dissolves. Remove from the heat, measure out 100ml of the syrup and combine this with the grenadine. Squeeze any excess water from the gelatine and stir it into the hot syrup and grenadine until dissolved. Leave to cool to room temperature. Once the jelly has cooled, stir in the pomegranate seeds.
Take the mousse cake out of the fridge and gently pour the jelly topping over, ensuring the mousse is evenly covered and the seeds are evenly distributed throughout. Chill for about 30 minutes, until set.
To serve, remove the sides of the tin, peel away the paper and arrange raspberries, pistachios and pomegranate seeds around the edge.