For the chocolate cake:

1. Preheat your oven to 350°F (175°C). Grease and line two 9-inch round cake pans with parchment paper.
2. In a large mixing bowl, sift together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.
3. Add the eggs, milk, vegetable oil, and vanilla extract to the dry ingredients. Beat on medium speed until smooth and well combined.
4. Add the sour cream and mix until fully incorporated.
5. Gradually pour in the boiling water, mixing gently. The batter will be thin—this is normal.
6. Divide the batter evenly between the two prepared cake pans.
7. Bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.
8. Allow the cakes to cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.

For the Nutella frosting:

1. In a large bowl, beat the softened butter with an electric mixer until creamy and smooth, about 2 minutes.
2. Add the Nutella and continue to beat until combined.
3. Gradually add the powdered sugar, 1/2 cup at a time, and beat until smooth and fluffy.
4. Add the heavy cream, vanilla extract, and a pinch of salt. Beat until the frosting is light and spreadable. If the frosting is too thick, add more heavy cream, one tablespoon at a time.

Assembling the cake:

1. Once the cakes have cooled completely, trim the tops to level them if needed.
2. Place the first cake layer on a serving platter or cake stand. Spread a generous layer of Nutella frosting on top.
3. Gently chop 6-8 Ferrero Rocher chocolates and sprinkle them over the frosting.
4. Place the second cake layer on top and frost the entire cake with the remaining Nutella frosting.
5. Decorate the top of the cake with the remaining Ferrero Rocher chocolates and optional chopped hazelnuts for added texture and crunch.

To serve:

1. Slice and serve the cake chilled or at room temperature for the best flavor and texture.
2. Enjoy the indulgent flavors of chocolate, hazelnut, and Nutella!