Mix together sugar, egg, milk, and salt sourdough until evenly combined. Gradually add in flour and knead until a rough dough forms. Add in butter and continue to knead until the dough is smooth and elastic. Cover and let rise overnight in a cool place (70-75F) for 8-10 hours.
Transfer the dough to a floured work surface. Using your hands, flatten the dough into a rough rectangle and then, using a rolling pin, roll out the dough into a rectangle about 9x12 with a thickness of around 1/2cm.
Combine brown sugar, cinnamon and corn starch in a bowl. Using a brush, brush the dough with butter. Evenly spread the cinnamon-sugar mixture on top and using your hands, lightly press it into the dough.
Tightly roll the dough from the short end-- like rolling up a scroll. If uneven, even out the log by gently press the spiral ends in. Using a knife, cut the log into 4 even pieces and place the rolls on a parchment lined baking dish. Cover and let rise for 2-3 hours.
Preheat your oven to 375F and bake for 15 minutes or until the roll starts turning golden brown. Remove from oven and let cool for about 20 minutes.
Combine the ingredients for the glaze until smooth. Pour the glaze over the warm cinnamon rolls from the center, coating the rolls. Or simply, dust with powdered sugar. Serve & Enjoy!