Preheat your oven to 350°F (175°C) and grease three 6-inch round cake pans, dusting them with flour. In a large mixing bowl, whisk together the dry ingredients: all-purpose flour, granulated sugar, Dutch-processed cocoa powder, baking soda, baking powder, and salt. In a separate bowl, combine wet ingredients: vegetable oil, buttermilk, eggs, hot water or coffee, and vanilla extract. Mix the two mixtures together until just combined, then divide the batter evenly among the pans and bake for 35-40 minutes. The cakes are done when a toothpick inserted in the center comes out clean. Let the cakes cool in the pans for 10 minutes before transferring them to a wire rack to cool completely.

While the cakes cool, prepare the Nutella Swiss meringue buttercream. In a heatproof bowl, combine egg whites and sugar, whisking vigorously over a pot of simmering water until the sugar completely dissolves. Once dissolved, remove from heat and beat the mixture on high speed until stiff peaks form. Gradually add in softened unsalted butter and Nutella, mixing until the buttercream is smooth and creamy.

For the rich Nutella ganache, place Nutella and heavy cream in a microwave-safe bowl. Heat in the microwave in 20-second intervals, stirring in between, until the mixture is completely smooth and glossy. If necessary, strain the ganache to remove any lumps, then allow it to cool at room temperature until it thickens slightly.

Once the cake layers are fully cooled, carefully slice each layer in half, creating six layers in total. Start with one layer and spread about ½ cup of the Nutella buttercream evenly on top. Drizzle a little ganache over the buttercream for added richness. Repeat the process for remaining layers, carefully stacking them one on top of the other.

To finish your stunning Nutella Cake, apply a crumb coat of buttercream around the exterior to trap any crumbs. Chill the cake in the refrigerator for about 30 minutes to set the crumb coat. Once chilled, generously frost the top and sides with the remaining buttercream. Decorate the edges with dollops of ganache for a beautiful drip effect. Voilà! Your Nutella Cake is ready to impress!