Line a 20 cm (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 into the bottom of the prepared pan to form an even base. Place the pan in the refrigerator to chill while preparing the filling.
In a heatproof bowl, melt the chopped dark chocolate over a double boiler or in short bursts in the microwave, stirring frequently until smooth. Set aside to cool slightly.
In a large mixing bowl, whip the heavy cream with powdered sugar and vanilla extract until stiff peaks form.
Gently fold the melted chocolate into the whipped cream until fully combined and smooth.
Pour the chocolate mixture over the chilled biscuit base in the springform pan, smoothing the top with a spatula.
Cover the cake with plastic wrap and refrigerate for at least 4 hours or until set.
Before serving, remove the cake from the springform pan and transfer it to a serving plate.
Garnish the top of the cake with chocolate shavings.
Slice and serve chilled.