Line a 20cm (8-inch) springform pan with parchment paper.
In a medium bowl, combine the crushed digestive biscuits and melted butter. Mix until the crumbs are evenly coated.
Press the biscuit mixture firmly and evenly into the bottom of the prepared pan to form the crust. Place in the refrigerator to chill while preparing the filling.
In a heatproof bowl set over a pot of simmering water (double boiler), melt the chopped dark chocolate until smooth, stirring occasionally. Remove from heat and let cool slightly.
In a separate bowl, whip the heavy cream with powdered sugar, vanilla extract, cocoa powder, and a pinch of salt until soft peaks form.
Gently fold the melted chocolate into the whipped cream mixture until fully combined and smooth.
Pour the chocolate filling over the chilled biscuit crust and spread evenly with a spatula.
Cover the cake with plastic wrap and refrigerate for at least 4 hours, or until set.
Once set, remove the cake from the springform pan, slice, and serve chilled.