Dark Chocolate Layer: 

1. Start by melting the dark chocolate in a heatproof bowl over simmering water. 
2. Once melted, remove from heat and let it cool slightly. In a separate bowl, whisk the egg yolk and half of the sugar until pale. 
3. Gradually fold the melted chocolate into the egg yolk mixture. In another bowl, whip the heavy cream until soft peaks form. 
4. In a clean bowl, beat the egg white with a pinch of salt and the remaining sugar until stiff peaks form. 
5. Gently fold the whipped cream and then the egg white into the chocolate mixture. 
6. Pour into serving glasses, filling only one-third. Refrigerate while preparing the next layer.

Milk Chocolate Layer: 

1. Repeat the same process as the dark chocolate layer, but using milk chocolate. 
2. Pour this mixture over the set dark chocolate layer in the glasses, filling another third. 
3. Refrigerate again.

White Chocolate Layer: 

1. Repeat the same process as the previous layers, but using white chocolate. 
2. Pour this mixture over the set milk chocolate layer in the glasses. 
3. Refrigerate for at least 4 hours, preferably overnight.

Serve: 

Before serving, garnish with chocolate shavings and fresh berries if desired.