In a large bowl, mix the sourdough starter, milk, and melted butter.
Add sugar, egg, and vanilla extract to the mixture and stir well.
In a separate bowl, combine flour, cinnamon, and salt.
Gradually add the dry ingredients to the wet mixture, stirring until combined.
Knead the dough on a floured surface for about 5 minutes.
Place the dough in a greased bowl and cover with a cloth.
Let it rise in a warm place for about 1 hour or until doubled in size.
Roll the dough into a rectangle and spread softened butter evenly over it.
Sprinkle a mixture of sugar and cinnamon over the buttered dough.
Roll the dough tightly and cut into 12 equal pieces.
Place the rolls in a greased baking dish and let them rise for another 30 minutes.
Bake in a preheated oven at 350°F (175°C) for 25-30 minutes.
Allow to cool slightly and drizzle with a glaze made from powdered sugar and milk.