1. In a large mixing bowl, combine the active sourdough starter, warm water, 1/4 cup granulated sugar, salt, and 3 tablespoons of melted butter. Stir until everything is well combined and the sugar and salt have dissolved.

2. Gradually add the 3 1/2 cups of all-purpose flour to the wet ingredients, mixing with a wooden spoon or your hands until a shaggy dough forms. Turn the dough out onto a lightly floured surface and knead for about 5-7 minutes until it becomes smooth and elastic.

3. Place the kneaded dough into a lightly oiled bowl, turning it once to coat. Cover the bowl with plastic wrap or a clean kitchen towel and let it rise in a warm spot for 2-3 hours, or until it has visibly increased in size and looks bubbly.

4. While the dough is rising, prepare the cinnamon filling. In a small bowl, combine the 2 tablespoons of softened butter, 2 tablespoons of granulated sugar, and 2 teaspoons of ground cinnamon. Mix until it forms a smooth, spreadable paste.

5. Once the dough has risen, gently punch it down to release the air. Turn it out onto a lightly floured surface and roll it into a rectangle, approximately 12×8 inches. Evenly spread the cinnamon butter mixture over the entire surface of the dough, leaving a small border around the edges. Starting from one of the longer sides, tightly roll the dough into a log. Pinch the seam closed to seal.

6. Carefully transfer the rolled dough log into a lightly greased 9×5-inch loaf pan. Cover the loaf pan loosely with plastic wrap or a clean kitchen towel and let it rise for another 45-60 minutes, or until it looks puffy and has nearly doubled in size.

7. Preheat your oven to 375 F (190 C) during the last 15 minutes of the second rise. Once preheated, remove the cover from the loaf pan and bake for 30-35 minutes, or until the top is golden brown and the internal temperature reaches 200-210 F (93-99 C) when checked with an instant-read thermometer.

8. Once baked, remove the loaf from the oven and let it cool in the pan for 10-15 minutes before carefully transferring it to a wire rack to cool completely. Let it cool fully before slicing.