Preheat oven to 350F. Grease three 8-inch round cake pans, and line the bottoms with parchment paper.
In a large bowl, whisk the flour, baking powder, baking soda and salt.
In another bowl, whisk the boiling water and cocoa powder until smooth.
Beat butter and sugar together on medium speed in a bowl of a standing mixer until light and fluffy. Beat in eggs, one at a time. Beat in buttermilk and vanilla extract.
Reduce the speed to low and beat in 1/3 of the flour mixture, followed by half of the chocolate mixture. Repeat with another 1/3 of the flour mixture and the remaining chocolate mixture. Beat in the remaining flour mixture until just incorporated.
Divide the batter and pour into the prepared pans, smoothing the surface with a spatula. Bake 20 to 25 minutes or until a toothpick inserted into the cake comes out clean.
Place the cakes (in the pans) on a wire rack to cool. Then invert the cakes onto a greased rack. Cool completely before frosting.
To prepare the frosting, in a standing mixer with a paddle attachment, beat Nutella and butter for a few minutes on medium speed. Add cooca powder and powdered sugar and beat until incorporated and you get to the right consistency.
To assemble the cake, place one cake layer on your serving plate and spread with 1/3 cup frosting. Sprinkle half of the chopped Ferrero Rocher and 1 tablespoon of chopped hazelnut. Place another cake layer on top, and gently press down a little bit. Spread with another 1/3 cup frosting, and sprinkle the same amount of Ferrero Rocher and hazelnut. Place the last layer of cake on top, smear frosting on the top and sides of the cake.
Gently press the hazelnuts into the sides of the cake.
Using a large star tip to pipe a decorative border with the frosting on the top edge of the cake. Place whole Ferrero Rocher on top, and sprinkle some more hazelnuts.
Cover and refrigerate. Remove from refrigerator an hour before serving.