For 6 pretzel buns:
1. In a bowl, add flour, salt and yeast.
2. Add water and milk to the bowl. Mix with a wooden spoon.
3. Add butter and start kneading the dough.
4. Knead until a ball forms and the dough is not sticky anymore.
5. Add the dough to a bowl, cover with plastic wrap and let it rest for an hour.
6. Put a liter of water to boil with 50g of baking soda.
7. Preheat the oven at 180°C/350°F.
8. Weigh the dough and divide it into 6 equal pieces. Shape the buns and use a blade to slash the buns a couple times.
9. When the water starts boiling, dip the buns for a minute in it and add it to a parchment paper covered baking tray.
10. Bake for 15 minutes at 180°C/350°F. They must be golden brown.
11. Let it cool completely before stuffing it with tomato, arugula, prosciutto and a generous slice of Echourgnac.