Cook chuck roast in Dutch oven on top of stove with enough water to cover the roast, until roast is tender, usually about 2 hours. Remove meat (shred when cooled). RESERVE broth in pan.
While beef is cooking mix flour, baking powder, and salt together. Cut in the margarine as for pie dough. Mix the beaten egg and water together and add to the flour and margarine mixture. Mix well. (If too sticky, add a bit more flour).
Knead dough on floured surface until smooth. Let stand covered for 30 minutes.
Roll out the dough very thin onto floured surface. Cut into squares. Put chopped potatoes in the pan with remaining broth,(you will need to add about 2 cups of water to the broth)and bring to boil. Drop squares of dough into the boiling broth. Cook over medium low heat until dough is done and potatoes are tender.