For the dough: Lightly grease large bowl with cooking spray. Mix water, sweetened condensed milk, oil, sugar, and egg in large measuring cup. Mix flour, yeast, and salt in bowl of standing mixer fitted with dough hook. With mixer on low, add water mixture. After dough comes together, increase speed to medium and mix until shiny and smooth, 4 to 6 minutes. Turn dough out onto heavily floured work surface, shape into ball, and place in greased bowl. (To make dough by hand: Combine dry ingredients in large bowl, make well in center of dry ingredients, add wet ingredients, and mix with wooden spoon until shaggy dough forms. Turn dough out onto heavily floured work surface and knead until shiny and smooth, about 10 minutes.) Cover bowl with plastic wrap and let rest in warm place until doubled in size, about 1 hour.
For the filling: Melt 1 tablespoon butter in large skillet over medium-high heat. Add beef and cook until just beginning to brown, about 6 minutes, breaking up any large clumps. Using slotted spoon, transfer beef to paper towel-lined plate.
Pour off all but 2 tablespoons fat from pan. Add onion and cook until softened, about 3 minutes. Add cabbage and toss until just beginning to wilt, 2 to 4 minutes. Return beef to pan and season with salt and pepper.
To assemble and bake: Adjust oven racks to the upper-middle and lower-middle positions and heat oven to 350 degrees. Coat 2 baking sheets with cooking spray. Divide dough into 8 equal pieces. Working on lightly floured work surface, roll each piece of dough into 7-inch circle. Place one dough round in deep cereal bowl and top with one slice of cheese. Spoon 3/4 cup filling over cheese and pinch edges of dough together to form bun. Transfer bun, seam side down, to prepared baking sheet. Repeat with remaining dough, cheese, and filling, placing 4 buns on each baking sheet. Cover buns with plastic wrap and let rise until puffed, about 20 minutes.
Bake buns until golden brown, about 20 minutes, switching and rotating position of baking sheets halfway through baking time. Brush buns with melted butter and serve.