Mix lukewarm milk, yeast, dough sugar, egg, and cooled melted dough butter. Add flour and salt and stir until a shaggy dough forms.
Knead 8–10 minutes by hand or 6–8 minutes with a dough hook until smooth and slightly tacky. Add bench flour sparingly rather than drying the dough.
Place in a lightly greased bowl, cover, and rise 60–90 minutes until roughly doubled. Grease the loaf pan while the dough rests.
Roll dough into a 9 x 18-inch rectangle. Brush with the tablespoon of filling butter and scatter mixed cinnamon and filling sugar, leaving a 1-inch clean border.
Roll from a short edge into a firm but not stretched log. Pinch the seam and ends closed, then place seam-side down in the pan.
Cover and rise 30–45 minutes until the loaf crowns about 1 inch above the rim. Heat the oven to 350°F (175°C) near the end.
Bake 35–40 minutes until golden and about 190°F (88°C) internally. Shield with foil if the top colors early, then cool in the pan 10 minutes.
Turn onto a rack and cool at least 1 hour before slicing. Cutting too early can compress the warm crumb and pull apart the spiral.
