First, prepare the chocolate ganache in order for it to set a little before you start piping it on the cookies.
Melt all the ingredients and mix well. Make sure you really use the amounts stated, to reach the right consistency - when it sets, it should be hard enough to be able to stack the cookies but still soft enough for that perfect bite. Let the ganache cool a little while you make and bake the cookies.
Preheat the oven to 180 degrees Celsius.
Knead all cookie ingredients with your hands into cookie dough that is not sticky (add some more flour if necessary). Form small balls from the dough (no bigger than Ferrero Rocher).
Dip the cookies into egg whites and then roll them in ground hazelnuts. Place on a baking tray covered with baking paper and press a hole in each with your finger.
Bake for 18 minutes. After you take the cookies from the oven, they will still be very soft, so let them cool a little before you transfer them to a plate. Let cool completely before decorating with chocolate.
Pipe chocolate ganache on each cookie and let them cool down and set.