• Prep the Pan: Line the bottom of a 9-inch springform pan with a round of parchment paper. Lightly grease the sides of the pan or line the collar with a strip of parchment paper for an effortless release.
• Make the Chocolate Syrup: In a small saucepan over medium-low heat, whisk together the sugar, cocoa powder, and water until dissolved.
• Add the butter and whisk until completely melted and smooth. Stir in the vanilla extract, remove from the heat, and let cool for 15 minutes.
• Break the Biscuits: Break the biscuits into small, rough, bite-sized pieces in a large bowl. Avoid crushing them into fine crumbs so you get distinct biscuit chunks.
• Combine: Pour the cooled chocolate syrup over the crushed biscuits. Mix thoroughly with a wooden spoon or spatula until every biscuit piece is coated.
• Press Into Pan: Transfer the biscuit mixture into the prepared springform pan. Press down firmly with the back of a measuring cup or offset spatula to pack it tightly into an even layer. Chill in the fridge for 30 minutes.

Make the chocolate ganache

• Place chopped chocolate in a heat-proof bowl. Heat the heavy cream in a saucepan until it simmers, then pour over the chocolate. Let sit for 5 minutes, then whisk from the center outward until smooth and glossy.
• Top, Chill & Unmold: Pour the ganache over the chilled biscuit base, smoothing it evenly. Cover with plastic wrap and refrigerate for at least 4 hours (or overnight) until set.
• Run a thin knife around the inside edge of the pan, unlock the springform collar, and top with chopped toasted walnuts before slicing.