Melt a stick of butter and stir in the flour and the 4 whole eggs.
Fill a piping bag with the batter and squeeze it into donut shapes on a baking tray. Bake at 350 °F for 10 minutes.
Heat the egg yolks and sugar in a saucepan. Fold in the cornstarch. Then add the milk, salt, the remaining stick of butter, vanilla extract, and Nutella, mixing well.
When the donuts and cream mixture have fully cooled, poke three holes in each donut, and pipe the Nutella cream into each hole.
Decorate the tops of the donuts with Nutella cream. As a finishing touch, dip the Nutella-glazed side of each donut into a plate of chopped hazelnuts.
