Very finely chop the dark chocolate into very small pieces (large chunks won't melt into the mixture as easily). Place the chopped chocolate into a large, heat-safe bowl.
In a saucepan, heat the cream, caster sugar, butter and vanilla over low heat, until steaming hot (but not boiling). Stir occasionally.
Carefully pour the hot cream over the chocolate and allow to sit for 1-2 minutes without stirring.
After 2 minutes, whisk or stir the mixture until the chocolate has melted and the mixture is smooth and glossy.
Pour the chocolate filling into the biscuit base and sprinkle with some salt flakes, to taste. Chill in the refrigerator for around 2 hours, until set.
Keep chilled until ready to serve. Slice and decorate with a dusting of cocoa powder (optional), fresh berries and double cream. Dig in.