Heat the cheese slices, milk, butter, white sugar and salt over low heat until melted into a smooth sauce that flows back halfway when drawn across the pan.
Cool the sauce until thick, then spoon it into a piping bag and set aside.
Knead all dough ingredients except salt and butter for 10 minutes, adding the salt and softened butter at the 4-5 minute mark.
Knead until a small piece stretches into a thin, translucent membrane without tearing - the windowpane test.
Divide into 12 pieces, round, rest 10 minutes, then roll each long, fold the ends into the center, fold again, pinch the seam and shape into a spindle.
Arrange on a lined tray and proof, covered, until doubled in size.
Dust with flour and cut a single slash 1-1.5 cm deep down the length of each bun.
Pipe the cheese sauce into the slash and across the top of each bun - the amount finishes exactly 12 buns.
Bake at 170°C top / 180°C bottom heat on the middle rack for 20 minutes until the cheese crust is deeply golden.
The sauce bakes into a glossy, milky crust with caramelized edges - rich and concentrated.
Cool until just warm. The crumb is soft and fine, the crust crisp and cheesy.