Preheat the oven to 300 degrees.
Prepare 2 loaf pans with vegetable oil or line with parchment paper.
Combine the sugar and cinnamon in a shallow dish. Cut the dough lengthwise into two pieces. Make 4 crosswise slits in each piece, taking care NOT to cut all the way through. Brush the loaves all over with butter, making sure to cover the creases. Roll each loaf in the cinnamon mixture, packing the cinnamon in the the creases with your hands. Place the loaves in the prepared pans and set aside in a warm place to rise for 30 minutes.
Bake the bread, turning the pan around once, for 35 minutes, or until the top is golden and a tap on the bottom of the pan yields a hollow sound.
Meanwhile, make the glaze. Vigorously stir together powdered sugar and water or lemon juice until the mixture is smooth and free of lumps. Turn the bread out onto a rack while it's still warm and drizzle with the glaze.