DOUGH - In the bowl of your stand mixer, add melted butter, sugar, eggs, vanilla, salt and mashed sweet potato and mix until well incorporated.
Add sourdough starter and mix until incorporated.
Add 4 cups flour, one cup at a time, and mix until a smooth dough forms.
Add 1/4 cup milk and the remaining 2 cups flour, 1/2 cup at a time and mix until a smooth dough forms. Add remaining milk, if needed (you're looking for a slightly sticky dough).
Place dough in a bowl and let rise, covered, until doubled in size (about 1-2 hours).
Punch down dough and roll out onto well floured surface.
Using a rolling pin, roll dough out into a rectangle about 1/4 inch thick.
FILLING - Brush melted butter all over the dough.
In a small bowl, mix together brown sugar and cinnamon. Sprinkle sugar mixture over melted butter and spread evenly with fingers.
Roll the dough, lengthwise into a tight log roll.
Cut dough into 12-16 equally sized rolls and place into cast iron skillet (sheet tray, pyrex pan or other cooking pan) leaving about 1/2-1 inch between each roll.
Cover with a tea towel and allow to rise until doubled in size (about 1-2 hours).
Preaheat oven to 350 degrees F and bake rolls, uncovered for 25-30 minutes, or until no longer doughy in the center.
FROSTING - In the bowl of your stand mixer, whisk together softened butter and powdered sugar. Add in sour cream, vanilla and sea salt and whisk until smooth and creamy.
Once cinnamon rolls are done baking, drizzle frosting over hot rolls for a thick glaze, or allow cinnamon rolls to cool and frost for a thin frosting.