Add the starter discard to a large bowl. Mix in warm milk and sugar, stirring until dissolved. Add bread flour, melted butter, and salt. Stir with a dough whisk or wooden spoon until a thick, shaggy dough forms.
Flour your hands, then finish mixing dough by hand until most of the flour is absorbed. *Don't worry if flour is on the sides or bottom of the bowl. Cover bowl with a clean, damp kitchen towel and let the dough rest for 30 to 45 minutes.
Stretch and fold the dough in the bowl:Grab a corner of the dough and stretch it 4 inches, then push it down into the middle of the dough. Turn the bowl and continue until you have done 4-6 stretches. Cover the bowl with a damp towel. Let the dough rise at room temperature for at least 8 hours, and up to 12 hours, until the dough doubles in size and there are bubbles on the top and bottom. **See notes below.
After the bulk rise, lightly flour your countertop. Remove the dough from the bowl and gently stretch it into a rectangle shape. Roll dough with a rolling pin until the rectangle measures 10" wide by 15" in length.
Add softened butter to a medium bowl. Using a pastry brush, apply butter evenly over the dough. Wipe out the bowl, then use it to combine sugar and cinnamon together. Sprinkle the mixture evenly over the butter and dough.
Working from the long end, roll up the dough into a log. Starting in the center of the log, use a chef's knife or waxed dental floss to cut the dough in half. Then cut each half roll into 6 pieces. This will give you 12 rolls total. Place the rolls into a greased 9"x13" casserole dish, leaving about 1" space between each cinnamon bun.
Cover dish with a clean kitchen cloth and let the rolls rise in a warm, draft-free area for 45 to 60 minutes, until doubled in size (they will almost touch one another).
Preheat oven to 350° F. Place pan of cinnamon rolls into the center of preheated oven. Bake for 25-30 minutes, until deep golden in color. Remove pan from oven and allow rolls to cool in pan for 10 minutes while you prepare the icing.
Icing
Add all icing ingredients to a medium bowl, and stir well to combine. Drizzle or brush icing over the warm rolls. Serve immediately.
Notes
Measuring Flour: Measuring your flour with cups is less reliable than using a food scale and weighing flour. To measure flour properly, don't scoop it! Use a small spoon to stir the flour, then use the spoon to fill the measuring cup with flour. Level off the top of the cup with a table knife. Do not compress or and tap down the flour.Rising TimeFor the best results, allow the dough to rise for at least 8 hours, or up to 12 hours. We usually make the dough in the evening and let it rise overnight.How long the bulk rise takes depends on your room temperature. Here are my guidelines: