On a wood or marble work surface make a well in the flour. Dissolve baking soda in milk at room temperature, then add to the flour. Add remaining ingredients and knead well with your hands until dough is firm and smooth.
Divide the dough into eight balls (called ""panett"" in local dialect), cover with a towel and set aside to rest for about 30 minutes.
Knead each piece of dough briefly and roll out with a rolling pin into 6-inch rounds, 1/8 inch thick. Heat a flat griddle (with no ridges) on the stovetop over medium heat.
When hot, place one dough disk on the surface, checking the piadina frequently and turning it once halfway through cooking.
Prick the dough with a fork to prevent too many air bubbles from forming. Prick evenly to prevent big bubbles from forming. This procedure should produce a flat piadina with its characteristic light and dark spots.