Step 1

Cook ¼ cup plus 3 Tbsp. milk and 2 Tbsp. bread flour in a small saucepan over low heat, whisking constantly, until thickened to a paste, 2–3 minutes. Transfer tangzhong to a small bowl and let sit until warm (it should resemble mashed potatoes), 5–10 minutes. Clean saucepan and reserve.

Step 2

Bring remaining ½ cup plus 1 Tbsp. milk to a gentle simmer in reserved saucepan over medium heat (watch carefully as milk tends to boil over). Pour milk into another small bowl and let cool until warm to the touch (about 110°). Stir in yeast and a pinch of sugar and let sit until foamy, 5–10 minutes.

Step 3

Combine egg, salt, remaining 2⅔ cups bread flour, and remaining ¼ cup sugar in the bowl of a stand mixer fitted with a dough hook. Add tangzhong and yeast mixture and mix on low speed until a shaggy dough forms. Add butter 1 piece at a time, mixing until fully incorporated after each addition before adding more. Increase speed to medium-high and continue to mix dough until tacky and slightly sticky, 8–10 minutes.

Step 4

Transfer dough to a lightly floured surface. Wet your hands to prevent dough from sticking, then pull and pinch ends of dough to form a smooth ball.

Step 5

Coat a large bowl with oil. Place dough in bowl and gently turn to coat. Arrange seam side down and cover bowl with plastic wrap. Let sit in a warm, draft-free spot until doubled in size, about 2 hours. (Alternatively, you can chill 8–12 hours).

Step 6

Roll out dough on a lightly floured surface to about an 18x10" rectangle. Using a small offset spatula, spread mayonnaise over surface of dough, leaving a ½" border around edges. Sprinkle 1 cup pork floss, then scallions and furikake evenly over mayonnaise. Roll up dough from a long side to form a tight log and pinch seams together to seal. Turn dough log seam side down and, using a sharp knife, trim 1" off each end; discard. Slice log into 8 equal portions.

Step 7

Coat a 10"-diameter cake pan with nonstick spray. Arrange rolls in pan, leaving a small gap between each roll. Cover pan with a damp kitchen towel and let sit in a warm, draft-free spot until buns are doubled in size, 1–1½ hours.

Step 8

Preheat oven to 350°. Whisk together egg and milk in a small bowl. Lightly brush tops of rolls with egg wash, then sprinkle sesame seeds evenly over.

Step 9

Bake rolls until golden brown, 25–28 minutes. Transfer pan to a wire rack and let rolls cool in pan until warm, or serve room temperature.Do ahead: Rolls can be baked 4 days ahead. Let cool completely; transfer to an airtight container and chill, or freeze up to 3 months. Reheat chilled buns in a 300° oven until soft and warmed through, about 5 minutes, or 30–45 seconds in a microwave. Frozen buns can be reheated on a baking sheet in a 350° oven 10–15 minutes.