Separate the egg white and yolk, put each in a different bowl.
Add sugar into the egg white and whisk until firm peaks. Set aside.
In another bowl, whisk whipping cream until stiff peaks. Lightly beat the egg yolk and add it into whipped cream. Gently fold to mix. Don't over mix. Set aside.
Melt chocolate in microwave. Add in melted butter, vanilla extract and salt. Mix everything together. Leave it to cool slightly before continuing.
Add 1 tablespoon of whipped cream yolk mixture into the chocolate mixture. Mix everything until well combined. Then pour the chocolate mixture into the bowl containing whipped cream yolk mixture. Gently fold to mix everything well. Don't over mix.
Add 1 tablespoon of egg white mixture and fold to temper the mixture. Then add it to the bowl containing egg white mixture. Gently fold to mix everything well. Don't over mix.
Divide equally into 2 glasses. Cover them with cling film and chill in the fridge overnight.