• Preheat the oven to 325°F. Spray an 8×8-inch square cake pan with cooking spray, line it with parchment paper, and spray again. Set aside.
• In a large bowl, whisk together the flour, sugar, cocoa powder, baking soda, baking powder, and salt until evenly combined.
• In a medium bowl, whisk the eggs, buttermilk, hot water, vanilla extract, and vegetable oil until smooth.
• Gently pour the wet ingredients into the dry ingredients and mix just until combined, ensuring no flour streaks remain. Use a rubber spatula to scrape the bottom of the bowl and make sure everything is evenly incorporated.
• Pour the batter into the prepared pan, and bake for 35-40 minutes, or until a toothpick inserted in the center comes out clean or with only a few crumbs.
• Let the cake cool completely for 15 minutes on the counter and then transfer it to the fridge for 45 minutes or the freezer for 20 minutes.

For the Hazelnut Filling

• Preheat the oven to 425°F. Line a sheet pan with parchment paper and spread the hazelnuts evenly across the pan. Sprinkle with salt and a small amount of oil.
• Bake for 10 minutes until roasted, then let them cool completely for about 10 minutes. Roughly chop the hazelnuts.

Make the Chocolate Ganache

• Once ready to assemble, prepare the ganache.
• Place the heavy cream in a medium microwave-safe bowl and heat until hot and slightly bubbly, about 30 seconds. Check and heat longer if needed.
• Remove from the microwave and add the chocolate. Let it sit for at least 3 minutes, covering it with plastic wrap to help speed up the process. Stir until smooth and fully melted.

Assembly

• Once the cake is completely cool, use a serrated knife to cut it in half horizontally to create two layers. Chilling the cake beforehand will make this easier.
• Spread about ¾ of a container of Nutella evenly over the bottom cake layer using an offset spatula, making sure to reach the edges. Sprinkle a generous amount of the roasted, chopped hazelnuts over the top and gently press them in.
• Place the second cake layer on top, then pour the chocolate ganache over the cake, spreading it evenly with an offset spatula. Sprinkle the remaining hazelnuts on top. If desired, finish with a pinch of flaky salt.