Preheat oven to 170C. Butter a 24cm-diameter springform cake tin and line the base and sides with baking paper. Process biscuits in a food processor until finely ground, then add cocoa, butter and 1 tsp salt and process to combine. Press evenly into the base and 2cm up sides of prepared tin, and bake until base is toasted and butter melts into crumb (15-16 minutes). Cool.
For ricotta filling, process ricotta, mascarpone, icing sugar and cocoa in a food processor until smooth. Bring coffee to the boil in a small saucepan. Squeeze excess water from gelatine, add to pan, then remove from heat and stir until dissolved (1-2 minutes). Cool for 5 minutes, then add coffee to ricotta mixture and process to combine. Fold in whipped cream and hazelnuts, then spread evenly over base, smoothing top, and refrigerate until set (4-5 hours).
For frosting, bring cream to the boil in a saucepan, then add chocolate and sugar and stir until melted. Transfer to an electric mixer and whisk until cooled and thickened to a spreadable consistency (5-10 minutes). Spread frosting over cake, and serve cut into wedges. The cake will keep refrigerated in an airtight container for up to four days (remove from fridge 15 minutes before serving).