Warm the milk until just slightly above body temperature, then mix in yeast and a teaspoon of sugar. Let it bloom until frothy, about 10 minutes.
In a large bowl, combine flour, remaining sugar, salt, and butter. Pour in the yeast mixture and egg, mixing until a soft dough forms.
Knead for about 10 minutes on a lightly floured surface until smooth and elastic. Consequently, the gluten will develop and ensure structure during baking.
Place the dough in a greased bowl, cover, and let it rise for 1–1.5 hours, or until doubled in volume. Meanwhile, prepare the cinnamon-sugar filling by mixing brown sugar and cinnamon.
Punch down the dough and roll it into a rectangle about 1 cm thick. Brush with melted butter and sprinkle the cinnamon mixture evenly. Therefore, every slice will have balanced layers of spice.
Roll tightly from the short side, pinch seams closed, and place in a greased loaf pan seam-side down. Cover and let rise again for 40–45 minutes.
Preheat the oven to 180°C (350°F). Bake for 30–35 minutes or until golden on top and hollow-sounding when tapped.
Cool in the pan for 10 minutes, then transfer to a rack. Finally, slice once completely cool to maintain clean, beautiful swirls.