Using a double boiler, melt the chocolate and set aside.
In a small bowl mix together the powdered icing sugar and cocoa.
In a large bowl, beat the cream and slowly add the icing sugar/cocoa mixture in a few additions. Continue beating until stiff peaks form.
In a separate large bowl, whisk together the cream cheese, crème fraîche, caster sugar and vanilla until well combined and glossy.
Add the melted chocolate to the cream cheese mixture and whisk through.
Add the whipped cream to the cream cheese mixture and fold to incorporate, avoiding overmixing.
Pour this mixture into the prepared crust and smooth.
Place into the refrigerator to chill for at least 4 hours, or ideally overnight.