In a food processor, pulse the Digestive Biscuit (15), Butter (6 tablespoon), and Dark Chocolate (85 gram) until crumbled.
Pour into the base of a 9-inch lined springform pan and press down evenly. Refrigerate for 30 minutes.
Melt remaining Dark Chocolate (400 gram) in the microwave in 30-second intervals, stirring between. Let it sit for about 10 minutes to cool.
Add the beaten Heavy Cream (2 1/2 cup) to the cooled chocolate in batches and fold gently to combine. Pour on top of the base and spread evenly. Refrigerate overnight.
Once the mousse is set, make the ganache. Add hot Heavy Cream (2 cup) to Dark Chocolate (450 gram) Let it sit for 1 minute. Stir together, starting in the middle, until completely combined.
Remove mousse from springform pan, leaving it on the base, and set over a rack. Pour warm ganache over chilled mousse cake and let it drip over the sides. Gently tap the tray to pop any air bubbles. Transfer to the fridge for 30 minutes to set.
Once chilled, cut into slices and serve!