Begin by melting the butter and warming the milk on the stove top or in the microwave. Then let cool to about 100° F.
Add the flour, sugar, and salt to a stand mixer and begin mixing on low.
While the mixer is running slowly, add the cooled but warm milk and butter, followed by the sourdough starter.
Once all the ingredients are combined, continue kneading the dough at a medium-low speed for 8-10 minutes.
Once the kneading is done, shape the dough into a ball and cover to let rise.
While the dough rises, make the filling by zesting 2 oranges and mixing the zest with 1/3 cup sugar. Rub between your fingers or use a fork to press against the side of a bowl until completely combined. Juice one of the oranges and mix the juice with 2 cups powdered sugar. It may be a little clumpy, but the sugar will dissolve while it rests. Place the filling and glaze in the fridge until they are needed.
Let the dough rise for 4-8 hours or until doubled in size. (The time will depend on the room's temperature. To speed it up, place it in the oven with the light on.)
When the dough has doubled in size, roll it out into a large rectangle (about 12"x18").
Top with zest filling and spread evenly over the dough. Cut the dough into 12 strips and roll them up individually. Alternately you can roll the rectangle of dough, then cut into 12 rolls with floss or a knife.
Cover and let the rolls do a second rise in a warm place until nice and puffy, about 2-4 hours.
Bake in the oven at 350° F for 25-30 minutes until they begin to golden around the edges.
Spoon the glaze over each roll and enjoy!
