Preheat the oven to 180°C fan.  Line a medium-sized square baking tin with parchment paper. You will also need four to six large ring moulds (approx 8cm wide). You will also need clear plastic (or office acetate), cut into 10cm wide strips.

For the chocolate base, melt the chocolate and butter in a bain marie (in a bowl set over saucepan of gently simmering water). Once melted, remove from the heat and allow to cool slightly. Stir the egg yolk through the chocolate.

In a large mixing bowl, use an electric whisk to whisk together the egg white, sugar and salt, until stiff peaks form. Add a spoonful of the egg white mixture into the chocolate to loosen it. Next fold the remaining egg white mixture through the loosened chocolate. Transfer to a square baking tin and bake in the preheated oven for 15 minutes until risen in the centre.

For the white chocolate mousse, melt the white chocolate in a bain marie.

In a separate bowl, set over a bain marie, whisk together the sugar, whole egg and egg yolk, whisking for five minutes until pale and doubled in size.  Remove from the heat and combine with the melted white chocolate, mixing until smooth. Set aside to cool for five minutes.

Soak the gelatine leaf in a bowl of cold water for five minutes to soften fully.  Take 50ml of the milk and heat to just below boiling in a small saucepan.  Immediately, take the soaked gelatine and squeeze out any excess water before stirring the gelatine into the hot milk. Allow to cool before adding to the white chocolate and egg mix. Stir in well.

Stiffly whip the cream and mix one third of the cream into the egg and white chocolate mixture to loosen it. Then lightly fold in the rest of the cream.

Use each ring to cut out a ring of chocolate cake (keeping it inside the base of the ring). Next, line the inside of each ring with strips of plastic. Divide the white chocolate mousse equally between the lined rings. Refrigerate and leave to set in the fridge for at least four hours or preferably overnight.

Release the mousse cakes from the rings, by running the tip of a sharp knife around the chocolate ring bases to first loosen them. Carefully slide them out of their rings and peel away the plastic. Decorate and serve chilled.

Variation 
To add a festive feel, decorate with pomegranate seeds and extra white or even dark chocolate curls.