Step 1

Whisk butter, sour cream, and salt in a medium bowl until smooth; set aside.

Step 2

DO AHEAD: Butter can be made 3 days ahead. Cover and chill.

Step 3

Combine yeast, 2 Tbsp. sugar, 2 tsp. salt, and 2 cups very warm (but not hot) water in a large bowl, whisking to dissolve. Let sit until foamy, about 5 minutes. Add 4 cups flour and mix with a wooden spoon until dough comes together in a shaggy ball with only a few dry spots of flour remaining. Cover with plastic wrap and let sit in a warm place until doubled in size, 50–70 minutes.

Step 4

Meanwhile, preheat oven to 400°. Toss potato with 1 Tbsp. vegetable oil and remaining ½ tsp. salt on a rimmed baking sheet and roast, tossing once, until potato is golden brown and soft, 15–20 minutes; let cool.

Step 5

While potato is roasting, chill bacon in freezer until firm but not frozen, 12–15 minutes. Transfer to a food processor and pulse until coarsely chopped. Cook bacon in a large skillet over medium heat, stirring occasionally, until brown and crisp, 15–20 minutes. Using a slotted spoon, transfer bacon to paper towels to drain.

Step 6

Once dough has doubled in size, gently punch down and transfer to a generously floured surface. Knead, adding more flour a tablespoonful at a time if dough is very sticky, until smooth (dough should be very soft, pliable, and slightly sticky).

Step 7

Divide dough in half. Flour surface generously. Cover 1 piece of dough with plastic so that it doesn’t dry out. Roll out the other piece of dough, adding more flour as needed to prevent sticking, to a 13" round about ¼" thick. Brush excess flour from dough, then brush with 1 Tbsp. sesame oil. Sprinkle with half of potato, bacon, cheese, and scallions.

Step 8

Roll up dough like a jelly roll. Pinch together seam and each end to seal, then gently squeeze and press from the center toward the ends to elongate the roll slightly and even out thickness.

Step 9

Starting from one end, coil roll (like a rope) to form a large spiral. Gently flatten spiral with your hand to form a 9" circle, sprinkling with lightly with more flour to prevent sticking if needed. Cover and repeat with remaining dough, potato, bacon, cheese, scallions, and 1 Tbsp. sesame oil.

Step 10

Whisk soy sauce, 1 ½ tsp. sugar, and 2 Tbsp. water in a small bowl until sugar is dissolved.

Step 11

Heat a 10" cast-iron skillet over medium. Add 1 Tbsp. vegetable oil, swirling to coat. Carefully place 1 spiral of dough in hot skillet (keep remaining spiral covered) and brush with soy glaze. Cover pan with a lid and cook until underside of bread is golden brown, 5–10 minutes (rotate skillet on burner as needed for even browning). Uncover and flip bread over. Brush top with soy glaze and sprinkle with 1 Tbsp. sesame seeds.

Step 12

Transfer skillet to oven and cook, uncovered, until underside is golden brown, 15–18 minutes (tent with foil if top gets too dark before bottom is golden brown). Transfer to a wire rack and let cool 15 minutes. Repeat with remaining round of dough, soy glaze, 1 Tbsp. sesame oil, and 1 Tbsp. sesame seeds.

Step 13

Cut warm bread into wedges and serve with sour cream butter.