There are many versions of soup dumplings, but this is my latest / favourite. It comes from a Saveur recipe for Cauliflower Soup.  In a small bowl, measure 1/3 cup flour; stir in ½ tsp kosher salt. Add 2 TB unsalted butter, cubed and chilled.  Work the butter into the flour with your fingers, until the butter bits are pea size.  Using a fork, mix in 1 egg. The mixture will be wet and "gloopy".  I like to make this before beginning the soup. I refrigerate it while making the soup and find that it is easier to work with once it has chilled and become more firm. (This can be made the day before and kept refrigerated in a covered container.)  The dumplings will cook in 3-5 minutes, so will be the last thing added to the soup.  Use one small spoon to scoop up some of the dough. Working over the pot of soup, use another spoon to push little bits of the dough off the spoon and into the soup. If the little bits don't fall off / in, dip the "pushing" spoon into the soup momentarily and that little bit will fall off.  If there is soup left the next day, you will find that the dumplings will have absorbed some of the liquid.  Add more water or broth to bring the soup back to the desired consistency.