Preheat oven to 325ºF (160ºC).
Place chocolate in heatproof bowl. In saucepan, bring 1 1/2 cups (375 mL) of the cream just to boil; pour over chocolate and stir until melted.
In separate bowl, whisk egg yolks with sugar; whisk in one-third of the chocolate mixture. Stir in remaining chocolate mixture, hazelnut liqueur and vanilla. Pour into six 6-ounce (175 mL) custard cups or ramekins; place in 13- x 9-inch (3.5 L) metal cake pan. Pour in enough boiling water to come halfway up sides of cups.
Bake in centre of oven until set around edge but still slightly jiggly in centre, about 20 minutes. Remove cups from pan; let cool on rack to room temperature. Cover and refrigerate until chilled. (Make-ahead: Refrigerate for up to 24 hours.)
In bowl, whip remaining cream. With piping bag or spoon, pipe onto centre of each serving. Garnish with hazelnuts.