parchment paper to line two non-stick 10” round cake tins
To make the cake:
Place egg whites in a clean grease-free bowl and add lemon juice and salt
Whisk until trebled in volume
Add in the cream of tartar and sugar
Sieve the flour and cocoa powder, twice and fold into the mixture spoon by spoon
Divide between the two lined cake tins and bake in pre-heated oven, 165°C for 30 minutes. Check that the cakes are cooked by gently putting a skewer or a knife into the middle of the cake and if the skewer comes out clean the cakes are cooked.
Allow cakes to cool in their tins until cold. Then turn out onto a wire rack and remove the parchment.
To make the filling:
In a saucepan put the cocoa powder, sugar and cornflour and whisk in the milk
Place over a medium heat, continue whisking until the mixture begins to thicken, when it starts to bubble cook for one more minute and remove from the heat.
Put one of the cakes onto a serving plate and pour half of the chocolate filling over it and place the other cake on top. Pour the remaining filling over the top and sides of the cake. Smooth with a palate knife and leave in the fridge for an hour before serving.