Preheat the oven to 350°F (175°C). Grease and flour a 9-inch round cake pan.
In a food processor, pulse the hazelnuts and flour until finely ground.
In a large mixing bowl, cream the butter and sugar until light and fluffy.
Add the eggs one at a time, beating well after each addition. Stir in the vanilla.
Gradually mix in the hazelnut-flour mixture, cocoa powder, and salt.
Fold in the milk until the batter is smooth.
Pour the batter into the prepared pan and bake for 30-35 minutes or until a toothpick inserted into the center comes out clean.
Let the cake cool in the pan for 10 minutes before transferring to a wire rack to cool completely.
Spread Nutella on top of the cooled cake and sprinkle with crushed Ferrero Rocher candies and chopped hazelnuts.