1. Make the milk lukewarm and dissolve the yeast in it. Stir the egg into it as well.
2. Add half of the flour to the milk and let it all rest for 10 minutes.
3. Now add the rest of the flour and knead for 20 minutes (manually) or 12 minutes on a slow setting in a food processor. When you are halfway through the kneading process you may add the butter, the pinch of salt and the Zùsto. 
4. Now let the dough sit for half an hour under a clean handcloth.
5. Then divide the dough into pieces of about 60 g.
6. Form them into balls and let them rest for another 10 minutes.
7. Now form these into sandwiches and place them on a greased baking tray.
8. Let them rise again under a dry cloth for 40 minutes.
9. Preheat the oven to 220 °C.
10. Coat the sandwiches with the beaten egg with a brush.
11. Bake the sandwiches for 10 minutes in the oven.

Enjoy!