1. Preheat your oven to 350°F (180°C) and prepare two 8-inch (20cm) round cake pans by greasing and flouring them.

2. In a large mixing bowl, beat the butter and sugar together until the mixture is light and fluffy, which should take about 3 minutes. Next, beat in the eggs one at a time, ensuring each egg is fully incorporated before adding the next.

3. In a separate bowl, whisk together the flour, corn syrup, and vanilla sugar. Gradually add these dry ingredients to the butter mixture, mixing until just combined.

4. Melt the chocolate in a double boiler or in the microwave in 30-second increments, stirring between each interval until the chocolate is smooth. Allow it to cool slightly, then fold it into the batter until well combined.

5. Divide the batter evenly between the prepared pans and smooth the tops. Bake for 30-35 minutes or until a toothpick inserted into the center of each cake comes out clean.

6. Remove the cakes from the oven and let them cool in the pans for 5 minutes before transferring them to a wire rack to cool completely.

7. Once the cakes are completely cool, spread a thin layer of marzipan on top of one of the cakes. Place the second cake on top and press gently to adhere. Melt the dark chocolate in a double boiler or in the microwave in 30-second increments, stirring between each interval until smooth. Pour the melted chocolate over the top of the cake, spreading it evenly to cover the entire surface.

8. Refrigerate the cake for at least 30 minutes to allow the chocolate to set before slicing and serving.