A sourdough starter and spelt flour lend an earthy aroma and beautiful flavor to luscious and pillowy cinnamon rolls.
Mix all ingredients together except for the butter in a stand mixer. Mix on 1st speed for about 5 minutes. Turn the mixer on 2nd speed and mix until the dough comes together and pulls away from the side of the bowl. Turn the mixer back to 1st speed and add butter to incorporate. Turn back to 2nd speed and mix until the dough pulls away from the side of the bowl and is shiny and elastic.
Form the ball into a round shape and cover with plastic wrap. Let ferment all day or overnight at room temperature (about 8-10 hours), so it doubles in size. A sourdough culture ferments dough much more slowly than dry yeast, so you won't have to worry about it over-fermenting.
Flatten the dough out to a 13 x 13 inch square on a lightly floured surface. Melt the 2 Tbsp butter and brush the surface of the dough. Mix the cinnamon and brown sugar together and sprinkle over the surface of the dough.
Roll the dough semi-tightly into a log and cut into 11-12 slices. Place them in a buttered casserole dish and cover.
Turn the oven on to 350 degrees and set the casserole dish on top of the stove so it has a warm place to proof. It will take about 40-45 minutes to proof, but it varies on the temperature of your house. The rolls will puff up about 30% and the spaces between them will be filled.
Bake in a 350 degree oven for about 40-45 minutes. You can puncture the inside of the dough with a knife to see if the dough is cooked through.
Mix the powdered sugar, 3 Tbsp milk, and vanilla together with a fork or whisk. Pour over the hot cinnamon rolls. Serve immediately.
The best way to ensure that your sourdough starter is ripe is to feed it 8-12 hours before you mix the dough. It should have risen and have a bubbly top.