This chocolate cake contains no butter or oil so is naturally lower in fat than a classic chocolate sponge. The addition of grated apple makes the sponge moist and its natural sweetness means you can afford to use less caster sugar.
Preheat the oven to 170C/330F/Gas 3.
Grease a 20cm/8in non-stick cake tin with a little olive oil cooking spray. Using an electric hand whisk, beat the egg whites with a pinch of salt until stiff peaks form when the whisk is removed from the bowl, then set aside.
In a separate bowl, whisk together the whole eggs, vanilla extract and caster sugar until pale, doubled in volume and the sugar has dissolved.
Fold in the apple, flour and cocoa powder with a metal spoon and then fold a third of the egg white mixture into the cocoa mixture. Once combined, fold this into the remaining egg white mixture.
Spoon the mixture gently into the cake tin and bake for 30 minutes, or until risen (when cooked the sponge should bounce back when pressed lightly). Transfer to a wire rack and leave to cool.
Once cooled, slice the cake in half horizontally to create two layers. Mash two-thirds of the raspberries and one tablespoon of icing sugar together roughly in a bowl and spread this mixture over the top of one of the pieces of cake. Top with the other piece.
Mix together the remaining icing sugar and enough water to bring it together to form a thin icing of drizzling consistency. Drizzle the icing over the cake, leave for 2-3 minutes to harden slightly and then decorate with the remaining raspberries.