Preheat your oven to 350 degrees.
Melt the butter, then let it cool until warm, but not hot.
Prepare your loaf pan by brushing with butter or coconut oil, or line with parchment paper.
In a large mixing bowl, whisk the melted butter, sugar, vanilla, eggs, and starter.
In a separate bowl, combine the dry ingredients: flour, salt, cinnamon, baking powder, and baking soda.
Gradually add the bowl of dry ingredients into the wet ingredients, then mix until it's just combined.
In a small bowl create the cinnamon swirl by mixing the cinnamon and brown sugar, then set to the side.
Add half of the dough into the loaf pan, then sprinkle 3/4 of the cinnamon sugar mixture over the top.
Add the remaining batter, followed by the rest of the cinnamon sugar.
Using a knife, make swirls through the batter to spread the cinnamon sugar throughout, then smooth over the top.
Bake at 350 for 60-70 minutes until golden brown. Check if it's baked through by pricking it with a sharp knife or toothpick. If it comes out clean, the loaf is done.
Cool for 20 minutes or so.
For the optional glaze, whisk together the powdered sugar, water, and vanilla until smooth.
Drizzle over the loaf and enjoy!