Combine the ingredients. In a large mixing bowl, combine the sourdough starter, flour, honey, butter, milk, and egg. Mix until a shaggy dough forms. Cover the bowl with a clean tea towel and let it rest at room temperature for about 30 minutes.
Knead the dough. After resting, add the salt to the dough. Knead the dough by hand, with a stand mixer, or in a bread machine until smooth and soft. If the dough feels too dry, add more milk, one tablespoon at a time. If it’s too sticky, add more flour, one tablespoon at a time. (If using a bread machine, select the DOUGH cycle and let the dough complete the rise in there, then just proceed to make the filling and shape.)
First rise. Place the dough in a clean, lightly oiled bowl. Then, cover it with a damp tea towel and allow it to rise in a warm place for 3-4 hours or until the dough has nearly doubled in size. The rising time may vary depending on the strength of your starter and the temperature of your room.
Prepare the filling. In a small bowl, combine the granulated sugar and cinnamon. Set aside.
Shape the dough. Grease a standard loaf pan with butter. On a lightly floured surface, stretch the dough into a long rectangle. Make sure the rectangle’s shorter edge is the same as your loaf pan’s length.
Add the cinnamon-sugar filling. Evenly sprinkle the cinnamon-sugar mixture over the entire surface of the dough.
Roll the dough. Starting from one short side, tightly roll the dough into a log, just like you would for a jelly roll.
Seal the edges. Pinch the seams and both ends of the rolled dough to seal them, making sure the filling stays inside during baking.
Second rise. Transfer the rolled dough to the loaf pan, positioning it with the seam facing down. Then, cover loosely with greased plastic wrap and let it rise in a warm place for about 1-3 hours, or until the bread rises 1 inch above the edge of the loaf pan.
Bake the bread. Towards the end of the rising time, preheat your oven to 350°F (175°C) with the rack positioned in the center. Once preheated, remove the plastic wrap from the loaf and bake for 40-50 minutes until golden brown and an instant-read thermometer inserted into the center reads 200°F (93°C).
Cool and serve. Remove the loaf from the oven and immediately transfer it from the pan to a cooling rack. Allow the bread to cool completely before slicing.
Notes
Rising times will vary depending on lots of factors: watch your dough, not the clock.