Cook Tangzhong over medium-low heat, whisking constantly, until thickened. Cool until warm.
Mix all dough ingredients except butter. Knead until you have meaningful gluten development, then gradually incorporate the butter. Knead until smooth, elastic and tacky.
First rise until substantially puffy, roughly 1½–2½ hours.
Roll to roughly 16×20 inches (40×50cm).
Distribute filling, roll from the long edge and cut into 12 rolls.
Put into a buttered 9×13in (23×33cm) pan.
Cover tightly and refrigerate overnight.
The next day, let them sit at room temperature until very puffy.
Pour warmed milk between the rolls (not over their tops).
Bake immediately at 350°F (175°C).
Start checking around 23 minutes. Aim for approximately 190–195°F (88–90°C) in the center and a lightly golden top.
Mix filling. Put it on while the rolls are still warm.