Mix starter, warm milk, and sugar in a large bowl
Add butter, eggs, and salt; mix well
Gradually add flour until shaggy dough forms
Knead 8-10 minutes until smooth
Cover; ferment 4-6 hours until doubled
Mix filling ingredients
Roll dough to 18x24 inch rectangle
Spread with butter and cinnamon-sugar mixture
Roll tightly; cut into 12 pieces
Place in greased 9x13 pan
Rise 2-3 hours or overnight in fridge
Bake at 375°F for 25-30 minutes
Mix frosting ingredients until smooth
Frost while warm