Preheat oven to 375 °F. Grease and line the bottom of a 9-inch pie pan with parchment paper.
Pulse chocolate chunks in a food processor with ¼ cup sugar to a fine grind, but not melted. Whip remaining sugar with eggs, zest and vanilla until pale and thick, about 5 minutes. Fold in chocolate, hazelnuts and melted butter just until combined. Pour into prepared pan and bake for 40 to 50 minutes, until a tester inserted in the center of the cake comes out clean. Let cool completely.
Heat cream to just below a simmer and pour over chopped chocolate. Stir until melted and stir in hazelnut liqueur. Pour over cake and garnish with hazelnuts.