Whisk together the milk and cocoa powder in a small saucepan, and heat gently until the milk is just about to come to the boil.

In a large mixing bowl, whisk together the egg yolks and caster sugar, and then add the cornflour.

Very slowly pour the chocolate milk into the egg mixture, continuously whisking as you do so to stop the eggs from scrambling.

Pour the mixture back into the saucepan and place over medium heat.

Keep whisking until the mixture thickens.

Place the chocolate crème pâtissière in a small bowl, cover with clingfilm and leave to cool in the fridge for at least 1 hour.

Place the egg whites into a large bowl, together with a few drops of lemon juice and a pinch of salt.

Whisk slowly until the mixture starts to foam.

Increase the speed and add the icing sugar and continue whisking until you have stiff peaks.

Melt the chocolate in a bain marie, or in a bowl over a pan of simmering water. Set  the chocolate aside to cool slightly.

Whisk the double cream in a large bowl until it thickens and forms soft peaks.

Pour the chilled chocolate crème pâtissière into a large bowl and gently beat to remove any lumps.

Stir the melted chocolate into the chocolate crème pàtissière.

Gently fold through the whipped egg whites, followed by the whipped cream.