In a large mixing bowl, combine the warm water and sourdough starter. Stir until well mixed.
Add the all-purpose flour, brown sugar, salt, and ground cinnamon to the bowl. Mix until a shaggy dough forms.
Knead the dough on a lightly floured surface for about 8-10 minutes, or until smooth and elastic.
Place the dough in a greased bowl, cover with a damp cloth, and let it rise in a warm place for about 1-2 hours, or until doubled in size.
In a small bowl, mix together the softened butter, granulated sugar, and vanilla extract to create the cinnamon filling.
Once the dough has risen, punch it down and turn it out onto a floured surface. Roll it into a rectangle about 1/2 inch thick.
Spread the cinnamon filling evenly over the dough, leaving a small border around the edges.
Starting from one long edge, carefully roll the dough into a tight log. Pinch the seams to seal.
Place the rolled dough seam-side down in a greased loaf pan. Cover and let it rise for another 30-45 minutes.
Preheat the oven to 350°F. Bake the bread for 30-35 minutes, or until golden brown and sounds hollow when tapped on the bottom.
Allow the bread to cool in the pan for 10 minutes before transferring it to a wire rack to cool completely.