Preheat your oven to 350°F (177°C). Prepare two 8-inch round cake pans by greasing them lightly and lining the bottoms with parchment paper for easy removal.

In a large mixing bowl, combine the all-purpose flour, cocoa powder, baking powder, and salt. Use a whisk to ensure the mixture is evenly blended.

In a separate bowl, whisk together the canola oil, sugar, Nutella, eggs, and milk. Stir until the mixture is smooth and consistent. Gradually pour in the hot water, mixing until the batter achieves a uniform consistency.

Gently fold the dry ingredients into the wet ingredients using a spatula. Mix until just combined to avoid overmixing. Divide the batter equally between the prepared pans, with approximately 730 grams of batter per pan.

Place the pans in the oven and bake for 35 to 40 minutes. Test for doneness by inserting a toothpick into the center of the cakes; it should come out with only a few moist crumbs.

Allow the cakes to cool in their pans for about 15 minutes. Then, carefully transfer them to a wire rack to cool completely. Once cooled, refrigerate the cakes for 1 to 2 hours to make frosting easier.

While the cakes chill, prepare the Nutella buttercream frosting. In a stand mixer or with a hand mixer, beat the softened butter and powdered sugar on low speed until they are thoroughly combined.

Add the Nutella, vanilla extract, salt, and 2 tablespoons of heavy cream. Mix on low speed until mostly incorporated, then increase to medium-high speed. Beat for 1 to 2 minutes to achieve a light and creamy texture. If the frosting is too thick, gradually mix in an additional 1 to 2 tablespoons of heavy cream to reach your desired consistency.

To assemble, spread a small dollop of frosting onto your cake stand or serving plate to anchor the cake. Place the first cake layer on the stand. Spread approximately ¾ cup of frosting evenly over the top, letting it slightly overlap the edges. Add the second cake layer and frost the top and sides with a thin layer to create a crumb coat. Chill the cake for 20 to 30 minutes to set the crumb coat.

After the crumb coat is firm, apply the remaining frosting generously to the cake. Smooth the sides using an icing knife or bench scraper. Create decorative swirls on top with any leftover frosting. Garnish with chopped hazelnuts if desired. Slice, serve, and savor every bite!