Prepare the oven and pan.
Preheat the oven to 340°F (170°C). Lightly grease a star-shaped cake pan with butter, reaching all the corners and points. Dust with cocoa powder and tap out the excess.
Mix the dry ingredients.
In a large bowl, combine the ground hazelnuts, granulated sugar, cocoa powder, dry chocolate pudding powder, cornstarch, baking powder, and salt. Break up any lumps.
Mix the wet ingredients.
In a separate bowl, whisk together the eggs, milk, orange juice, oil, vanilla sugar, and orange zest until well combined.
Combine and fill the pan.
Add the dry mixture to the wet ingredients. Stir until evenly combined, scraping the sides and bottom of the bowl. Transfer the batter to the prepared pan, spread it into each point, and smooth the surface.
Bake.
Bake for approximately 40 minutes, until a toothpick inserted into the center comes out without wet batter.
Cool completely.
Remove the cake from the oven and let it cool completely in the pan. Carefully turn it out onto a serving plate, supporting the star points as you release it.
Make the glaze.
Gently melt the chopped chocolate with the oil, stirring until smooth. Pour over the cooled cake and spread toward the edges.
Decorate.
Sprinkle with chopped toasted hazelnuts while the glaze is still soft. Allow the glaze to settle before slicing into 15 servings.