Line a 20cm (8-inch) springform cake pan with parchment paper.
In a medium bowl, combine the crushed digestive biscuits and melted butter until the mixture resembles wet sand.
Press the biscuit mixture firmly and evenly into the bottom of the prepared cake pan to form the base. Place in the refrigerator to chill while preparing the filling.
In a heatproof bowl, melt 300 grams of dark chocolate over a double boiler or in the microwave in 30-second intervals, stirring until smooth. Set aside to cool slightly.
In a large mixing bowl, whip 200 milliliters of heavy cream with powdered sugar and vanilla extract until soft peaks form.
Gently fold the melted chocolate and cocoa powder into the whipped cream until fully combined and smooth, creating a chocolate mousse.
Pour the chocolate mousse over the chilled biscuit base and smooth the top with a spatula. Refrigerate for at least 4 hours or until set.
To prepare the ganache, heat 100 milliliters of heavy cream in a small saucepan over medium heat until it just begins to simmer. Remove from heat and pour over 100 grams of chopped dark chocolate in a bowl. Let sit for 2 minutes, then stir until smooth and glossy.
Allow the ganache to cool slightly until it thickens but is still pourable, then pour it evenly over the set mousse layer.
Return the cake to the refrigerator and chill for an additional 1 hour until the ganache is set.
Before serving, carefully remove the cake from the springform pan and peel off the parchment paper. Slice and serve chilled.