Make the dough in the evening and let rise overnight.
Combine the melted butter and milk in a small bowl. Cool slightly before using.
Add the egg, sourdough starter, and sugar to the bowl of a stand mixer fitted with the paddle attachment. Mix to combine.
With the machine running, slowly pour in the milk mixture. Add the flour and salt. Continue mixing until a rough, sticky dough forms, about 1 minute.
Cover with a damp towel and let rest for 30 minutes.
Switch to the dough hook. Knead on medium-low speed for 6-8 minutes.
Transfer the dough to a medium-size bowl coated in butter. Cover with plastic wrap. Let rise overnight until double in size.
In the morning, line a 9-inch springform pan with parchment paper.
Lightly oil and flour your countertop. Coax the dough out of the bowl and gently pat into a rough rectangle. Let rest for 10 minutes.
Dust the dough with flour and roll it into a 16 x 12-inch rectangle.
Prepare the cinnamon-sugar filling and spread it onto the dough.
Roll the dough into a log and cut into 2-inch sections.
Place the rolls into the lined pan and let rest for 1-2 hours.
Preheat oven to 350 F. Bake for 35-40 minutes.
Remove from the oven and cool in the pan for 15 minutes.
Make the glaze and top the rolls before serving.
