Step 1: Prepare the Cake Batter

1. Preheat your oven to 350°F (175°C) and grease a 9-inch round cake pan. Line the bottom with parchment paper for easy removal.
2. In a large mixing bowl, combine the dry ingredients: flour, sugar, cocoa powder, baking soda, baking powder, and salt. Whisk well to combine.
3. In a separate bowl, whisk together eggs, vegetable oil, buttermilk, and vanilla extract until smooth.
4. Gradually add the wet mixture to the dry ingredients, stirring until well combined. Slowly add the hot water and mix until the batter is smooth.

Step 2: Create the Chocolate Swirls

5. Divide the batter into three equal portions. In one portion, fold in the melted white chocolate. In the second portion, fold in the melted milk chocolate. In the third portion, fold in the melted dark chocolate.
6. Pour the batters into the prepared cake pan in alternating dollops to create a swirled effect. Use a skewer or knife to gently swirl the batters together, creating a marbled pattern.

Step 3: Bake the Cake

7. Place the cake pan in the preheated oven and bake for 30-35 minutes or until a toothpick inserted into the center comes out clean.
8. Once baked, remove the cake from the oven and allow it to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.

Step 4: Prepare the Hazelnut Ganache

9. In a small saucepan, heat the heavy cream over medium heat until it begins to simmer (do not let it boil).
10. Remove the saucepan from the heat and stir in the hazelnut spread and chocolate chips. Whisk until the mixture is smooth and glossy.
11. Allow the ganache to cool slightly until it thickens to a spreadable consistency.

Step 5: Assemble and Decorate the Cake

12. Place the cooled cake on a serving platter. Pour the hazelnut ganache over the top, using a spatula to spread it evenly over the sides.
13. Garnish with white chocolate shavings, milk chocolate curls, dark chocolate chunks, and chopped hazelnuts for a stunning finish.