Peel the potatoes, rinse, grind in a blender, drain the excess liquid. Finely chop the lard or mince it. Boil the water and leave to cool.
Sift the flour into a deep plate. Add salt and mix. Beat in the egg. Gradually add water. Knead the dough thoroughly. Leave for 10-15 minutes.
In a separate bowl, mix potatoes with lard. Salt and pepper.
Roll out the dough into a thin layer. Using a mold or mug, cut out circles, add the filling, wrap, and secure with your fingers.
Pour water into a saucepan, add salt, place over medium heat, and cook until boiling. Add the dumplings, reduce the heat, and cook for another 7-8 minutes.
Serve the dumplings hot on individual plates, adding sour cream if desired.