For the crust, pulse hazelnuts in food processor with sugar until finely ground. Add flour, cocoa and salt. Pulse in the butter, until soft dough forms (crumbly). Press into buttered 9-inch springform pan, bottom only. Poke with fork all over and bake in a 375 degree oven for 18-20 minutes. Cool completely.
For the filling, pour cold water in small saucepan. Sprinkle with gelatin, allow to bloom (absorb water) for 2 minutes. Heat saucepan on low and whisk gelatin water for several minutes until completely absorbed and dissolved. Remove from heat.
In mixing bowl, beat gelatin mixture with Nutella and mascarpone cheese. Set aside.
In mixing bowl, beat whipped cream with cocoa and sugar until soft peaks form (several minutes). Fold in nutella mixture. Pour over cooled crust and refrigerate for several hours (or overnight).
To remove springform pan, use plastic knife and run along edges. Release spring and lift up. Tart should be set up nicely on insert.
When ready to serve, place heavy cream and chocolate in small saucepan. Heat on medium-low and stir until smooth. Drizzle over slices of tart. Enjoy!