Preheat the oven to 375 degrees F. Spray an 8-inch round cake pan with nonstick butter spray and line bottom with parchment.
Put the chocolate, butter, vanilla and hazelnut liqueur in a large bowl on top of a double boiler. Stir the mixture frequently until combined and smooth. Remove the chocolate from heat and put the bowl into an "ice bath" to cool the chocolate.
Cook's Note: This step is important so the eggs will not cook when added to the chocolate. Mix in the sugar and cocoa powder. Stir in the eggs and pour into the prepared cake pan. Put the cake pan on a sheet tray and bake for 20 to 25 minutes.
Remove the cake from the oven and invert onto a rack over a sheet pan lined with parchment paper. Pour the ganache over the cake. Allow the cake to cool and set for 10 minutes before serving. Garnish with fresh berries and powdered sugar.
Put the chocolate into a large bowl. Heat the cream in a small pot, over low heat, until hot to the touch. Pour the hot cream over the chocolate. Mix well with a rubber spatula until the chocolate melts.