In a bowl, whip the butter and the sugar with an electric mixer. Then add the eggs one by one, including the yolk, add the milk and a teaspoon of vanilla extract until the dough is smooth and consistent. Sift the flour and then the baking powder and salt. Combine the powdered mixture by slowly folding from bottom to top. Add the hazelnuts. Place a fluted paper cup in a muffin mold and pour in the mixture. Be careful not to fill it more than 3/4 of the way.
For the butter cream:
In a bowl, whip the butter and the powdered sugar until smooth and creamy. Decorate the cupcakes with a pastry bag fitted with a star-tipped piping nozzle. Sprinkle the hazelnuts and top it all off with a Ferrero Rocher.