Put potatoes in a medium saucepan and cover with hot water. Cook over high heat until tender (about 12 minutes).
Meanwhile, heat oil in a large, heavy casserole. Add chicken and cook over high heat until lightly browned, 4 minutes. Add onion and jalapeño and cook, stirring, until the onion is softened (about 4 minutes). Add 3 T of the fish sauce and the stock and bring to a boil. Cover and cook over medium heat until the chicken is just cooked through (about 10 minutes).
Using a slotted spoon, transfer potatoes to a ricer and press into a bowl. (or you can mash the potatoes.) Reserve the potato water. Add the 2 tablespoons of flour, egg yolk, chives and 1/2 teaspoon of salt to the potatoes and stir until a stiff dough forms.
Turn the dough out onto a heavily floured board and divide it in half. Roll each piece into a 1/2 -thick rope. Cut the ropes into 1 pieces.
Add the bok choy to the stew and cook until crisp-tender, about 5 minutes. Add the cornstarch mixture and cook, stirring, until thickened, 1 minute.
Return the potato water to a boil and add the dumplings. Cook over high heat until they rise to the surface, then simmer for 2 minutes. Using a slotted spoon, transfer to the chicken stew. Add the basil and the remaining 1 tablespoon of fish sauce and simmer the stew for 2-3 minutes.