A cinnamon sugar swirl inside of the most delicious, soft and flavorful bread! We love this plain, toasted + slathered with butter, or as French toast.It's best enjoyed 24 hours after baking, since the gooey cinnamon swirl will start to affect the surrounding bread's texture after that, but leftovers will keep for up to 3 days.
Combine the milk, starter, butter, egg, sugar, and salt in the bowl of a stand mixer using the paddle attachment until smooth. Add in the flour and mix until no dry bits remain, stopping to scrape the sides of the bowl as needed. Cover the bowl with plastic wrap and rest for 30 minutes.Attach the dough hook and knead on LOW speed for 6-8 minutes, until the dough is smooth and pulls away from the sides of the bowl. (It will still stick on the bottom - this is okay!) If it's too sticky, dust with more flour. Cover and rest until doubled in volume, about 8-12 hours. Generously butter a loaf pan. Combine the sugar, cinnamon, and flour in a small bowl.Turn the dough out onto a lightly floured work surface. Gently flatten it out with your fingertips to get out any large air bubbles. Roll and stretch the dough into a roughly 9-by-20 inch rectangle. Check frequently to make sure it isn't sticking to your work surface! Add more flour as needed.Brush the surface with water, then sprinkle on the cinnamon sugar, leaving a 1" boarder on one of the shorter edges. Starting on the opposite end, roll the dough into a tight log. Pinch the ends to seal and tuck under the loaf. Gently pull the loaf toward yourself to tighten it up, then place it - seam side down - into the loaf pan. Cover with a towel and allow it to rise for 1-2 hours, until it has risen over the top of the pan by about 1 inch. Bake for 35-40 minutes in a 375 degree oven, until golden brown. Cool in the pan for 10 minutes, then (carefully!) invert to release, and cool for at least an hour on a wire rack before slicing.