Bloom yeast in milk. Stir mixture.
When mixture starts to foam, add sourdough starter, eggs, butter, and sugar.
Mix until thoroughly combined.
Add flour, salt, and cinnamon and mix until just combined.
Leave dough to rise in the refrigerator overnight.
The next day, split the dough in half. Roll out to ~9x16" rectangle.
Spread half the softened butter evenly onto the dough.
Sprinkle half of the brown sugar and half of the cinnamon onto the dough. roll dough into a log and cut into 12 rolls.
Repeat dough rolling, butter spreading, sugar/cinnamon spreading with the second half of the dough.
Place cinnamon rolls in a 9x16" pan, leaving space between the cinnamon rolls.
Leave in the refrigerator to rise again overnight.
The next day, remove from the refrigerator ~30 minutes prior to baking for cinnamon rolls to come to room temperature.
Bake at 350F for ~20 minutes, when tops are golden brown. Serve warm!