Mix 3 cups all-purpose flour, buttermilk, sourdough starter, 2 tablespoons cubed butter, and white sugar in a glass bowl. Cover and set in a warm area until double in size, 8 to 12 hours.

Preheat the oven to 350 degrees F (175 degrees C).

Combine baking powder, salt, and baking soda in a small bowl. Add to flour mixture.

Knead dough until it is soft but no longer sticky, adding flour as needed. Roll dough with floured rolling pin until it is 1/4 to 1/2-inch thick.

Combine 6 tablespoons melted butter, brown sugar, and cinnamon until it has the consistency of a paste. Spread sugar paste evenly on rolled dough.

Roll up dough into a cylinder shape. Cut dough into 1/2-inch pieces using dental floss or a serrated knife. Arrange rolls into a cast iron skillet.

Bake cinnamon rolls in the preheated oven until golden brown, 20 to 30 minutes.