In a large bowl, whisk 375 grams of all purpose flour with 1 teaspoon of salt and 2 teaspoons of baking powder
Use a stiff fork to work 50 grams of shortening into the flour mixture until no large pieces remain (alternatively, melt the shortening and drizzle over the flour)
Form a well in the flour mixture and add 200 grams of warm milk into the well
Begin working the flour into the warm milk until a soft dough begins to form
Transfer the dough onto a floured surface and knead the dough for about 3 minutes, adding a pinch of flour if the dough feels sticky
Divide the dough into 6 equal portions and gently form into round balls
Cover the dough with a kitchen towel and let the dough rest for 10 minutes on the floured surface
Place a flat skillet or griddle over medium heat
Working with one portion of dough at a time, roll the dough out to a large circle approximately 8-9 inches across and less than 1/4 inch thick
Use a fork to poke holes in the dough
Transfer the dough onto a dry, preheated skillet
Cook for a minute or two on the first side and once the bottom is lightly charred, flip and cook for another minute or two on the other side
Transfer the cooked piadina into a large kitchen towel and wrap it to keep it warm while you work with the other portions of dough
Repeat the process with the other portions of dough and stack them all in the towel
To fill the piadina, line one half of a warm piadina with thinly sliced fresh mozzarella
Top with thinly sliced prosciutto and arugula
Fold the piadina in half, slice in half and enjoy!
Store left over piadina wrapped tightly at room temperature