For crust, preheat oven to 350F (180C) and grease a 9-inch (2.5 L) springform pan. Pulse graham crumbs, hazelnuts and brown sugar in a food processor. Add melted butter and vanilla extract and pulse until an even texture. Press into bottom of prepared pan and bake for 7 minutes. Allow to cool while preparing filling.
For filling, reduce oven temperature to 325F (160C). Heat cream to just below a simmer. Pour hot cream over chopped chocolate and whisk gently until smooth and set aside. In a bowl using electric beaters or with a mixer fitted with the paddle attachment, beat cream cheese until fluffy, scraping the sides of the bowl often. While beating, slowly pour in sugar, again scraping the bowl often. Beat in cornstarch. By this time the filling should be looking a little more fluid. Slowly pour in hazelnut liqueur and beat in eggs, one at a time. Pour chocolate mixture into cheesecake filling and blend well. Scrape filling into pan and bake for 30 minutes. Without opening the oven door, turn off the oven and leave cheesecake in another 30 minutes. Remove, let cool to room temperature, then chill overnight.
To serve, remove cheesecake from pan, dust top of cheesecake with cocoa powder and decorate with hazelnuts.