Preheat the oven to 180°C (approximately 350°F). Grease the springform pan with butter and dust with flour.
Coarsely chop the chocolate and melt in a double boiler. Allow to cool slightly. Beat the butter, sugar, vanilla sugar and salt with an electric mixer until smooth and creamy. Add the eggs, one at a time and mix until smooth. Mix together the flour, cocoa powder, ground hazelnuts and baking powder. Stir the flour mixture into the egg mixture alternately with the orange juice. Fold in the melted chocolate and pour into the prepared pan. Bake for 45 minutes, until a toothpick inserted comes out clean. Allow to cool for about 10 minutes, carefully release from the pan and let cool on a wire rack.
Heat the orange marmalade in a small saucepan. Slice the cake in half horizontally and brush with the orange marmalade. Garnish with chocolate curls, slice and serve.