Recipe adopted from The New Best Recipe. [Recipe makes (1) 9-inch loaf].
1/2 cup whole milk
4 tablespoons unsalted butter, cut into 1/2" pieces
1/2 cup water, warmed to 110° F
2 1/4 teaspoons instant dry yeast
2 1/3 ounces (~1/3) cup sugar
2 large eggs
1 1/2 teaspoons salt
18 3/4 ounces (~4 cups minus 1 tablespoon) all-purpose flour
1 3/4 ounces (~1/4 cup) sugar
5 teaspoons cinnamon
Milk, for brushing
1 large egg
2 teaspoons milk
In a small saucepan (or microwave), heat the butter and milk until the butter is melted; let cool back to 110° F. In the bowl of a stand mixer, add the warm water, yeast, sugar, and eggs and mix low with a beater attachment until combined. Add the salt, milk mixture, and half of the flour. Mix on medium for one minute. Add the remaining flour and mix on medium for 10 minutes, using the hook attachment.
Transfer the dough to a lightly floured surface. If the dough is still sticky to the touch, knead in up to 1/2 cup of additional flour by hand. Knead into a ball and transfer to a lightly greased bowl. Cover with plastic wrap and let sit for 2 hours, or until doubled in size.
With a floured hand, gently press down on the dough to deflate. Transfer the dough to a counter and let rest for 10 minutes. While you're waiting, grease the insides of a 9" x 5" bread loaf pan. Also, combine the sugar and cinnamon in a small bowl.
Shape the dough into a rectangle, and then roll out to an 18" x 8" rectangle, with the shorter side closest to you. Brush the dough liberally with milk, and then sprinkle the cinnamon sugar filling evenly over the dough, making sure to leave a clean 1-inch border along the edge closest to you. If it doesn't seem like the milk has absorbed all of the mixture, use a spray bottle and mist the top of the filling with water or milk until damp. Starting with the short end away from you, roll the dough into a tight log, making sure the log doesn't exceed 9-inches in length. Once you get to the 1-inch border, finish rolling and then seal the end by pinching the dough along the seam. In addition, stretch the tops of each end down to the bottom and seal, to prevent any cinnamon from escaping the loaf. Carefully transfer the dough to the greased pan, seam side down. Loosely cover with plastic wrap and let rise until the dough comes 1-inch above the top of the pan, roughly 1 hour.
When the dough looks like it's halfway through the final proofing, preheat the oven to 350° F and adjust an oven rack to the middle position.
Once ready, gently brush the eggwash on top of the loaf. Transfer to the oven and bake for 30 to 35 minutes, or until an internal temperature of 185 to 190° F is met. Transfer the loaf to a wire rack and let cool for at least an hour before slicing into.