Line base of a deep 18 cm round springform pan with baking paper.
Add butter, sugar, cocoa powder and water into a small pot and bring it to boil. Simmer the syrup for about 5 minutes. Remove from stove and add vanilla extract, let the mixture cool down for 10 minutes.
Place the biscuit pieces and roasted nuts in a big bowl. Pour the chocolate syrup over the biscuit, nut mixture and stir until it is well coated with the chocolate syrup.
Place the mixture into the prepared pan, Level up and press down with a spoon to make sure the base is tight. Cover and refrigerate the cake for about 1 hour.
Meanwhile prepare chocolate ganache by melting chopped chocolate and fresh cream in a heat proof bowl over simmering water, stir until smooth. Let it cool slightly.
Pour the chocolate ganache over the cake and spread it with a spatula. Refrigerate the cake for 3 - 4 hours or overnight.
NOTE - I used Mc Vities original digestives, Hershey's natural unsweetened cocoa and Ghirardelli bitter sweet chocolate 60% cocoa in this recipe.