Heat a large skillet over medium-high heat; cook and stir ground beef and onion until beef is crumbly, evenly browned, and no longer pink. Drain and discard any excess grease. Stir in cabbage and season with 1 teaspoon salt. Continue to cook and stir until cabbage is tender, about 20 more minutes. Set mixture aside to cool.

Mix warm water and butter in a large bowl. Combine dry milk powder, sugar, 1 1/2 teaspoons salt, and yeast in another bowl; stir into butter mixture. Add egg and gradually stir in flour, 1/2 cup at a time, until dough is smooth.

Roll dough out to 1/4-inch thick on a lightly floured surface; cut into 4-inch squares. Scoop beef mixture into each dough square. Bring corners of a dough square together at the top and pinch corners together to seal. Place on a baking sheet, seam-side down. Repeat with remaining beef mixture and dough. Cover rolls with a damp cloth and allow to rise for 20 minutes.

Preheat the oven to 375 degrees F (190 degrees C).

Bake in preheated oven until golden brown, about 20 minutes.