Lightly brush a 20cm (base measurement) springform pan with oil to grease. Line the base and side with plastic wrap. Place 6 sponge slices around the side of the pan. Place 3 sponge slices over the base and cut the remaining sponge slice into smaller pieces to fill in the gaps over the base.

Place the chocolate in a large heatproof bowl. Place 80ml (1/3 cup) of the cream in a small saucepan over medium heat and cook, stirring, for 3 minutes or until it just comes to the boil. Pour the cream over the chocolate. Use a metal spoon to stir gently until the chocolate melts and the mixture is smooth. Quickly stir in the egg yolks and vanilla.

Place the water in a small heatproof bowl. Sprinkle with the gelatine. Place the bowl in a small saucepan. Add enough boiling water to the saucepan to come halfway up the side of the bowl. Use a fork to whisk until the gelatine dissolves. Stir into the chocolate mixture.

Use an electric beater to beat the remaining cream in a bowl until soft peaks form. Use a clean, dry electric beater to beat egg whites in a separate bowl until soft peaks form. Use a metal spoon to fold the cream and egg whites into the chocolate mixture. Pour into the prepared pan. Cover and place in the fridge for 6 hours or until firm.

Place the cake on a serving plate. Use an electric beater to beat the extra cream and icing sugar in a bowl until soft peaks form. Spread over the top of the cake. Top with the Violet Crumble to serve.