1. 1. Combine Wet Ingredients: In a large mixing bowl, whisk together the active sourdough starter, warm milk, 1/4 cup granulated sugar, egg, 1/4 cup melted butter, and vanilla extract until well combined. The mixture should look slightly frothy.

2. 2. Add Dry Ingredients: Gradually add the all-purpose flour and salt to the wet ingredients. Mix with a wooden spoon or your hands until a shaggy dough forms.

3. 3. Knead the Dough: Turn the dough out onto a lightly floured surface and knead for 5-7 minutes. The dough should become smooth and elastic, bouncing back when gently poked. If it’s too sticky, add a tiny bit more flour, a tablespoon at a time.

4. 4. First Rise: Place the kneaded dough in 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 place for 2-3 hours, or until doubled in size. You’ll know it’s ready when it looks puffy and passes the “poke test” (an indentation remains when you gently poke it).

5. 5. Prepare the Swirl and Shape: While the dough rises, prepare the cinnamon swirl filling by mixing 2 tablespoons softened butter, 2 tablespoons granulated sugar, 2 teaspoons ground cinnamon, and 1/4 cup packed light brown sugar in a small bowl until it forms a paste. Once the dough has risen, gently punch it down and roll it out on a lightly floured surface into a rectangle, approximately 12×18 inches. Spread the cinnamon filling evenly over the dough, leaving a small border around the edges.

6. 6. Roll and Second Rise: Starting from one of the longer sides, carefully roll the dough into a tight log. Pinch the seam closed to seal. Place the rolled dough into a greased 9×5-inch loaf pan. Cover loosely with plastic wrap or a kitchen towel and let it rise again in a warm place for another 30-45 minutes, or until visibly puffy.

7. 7. Bake the Bread: Preheat your oven to 375 F (190 C). Bake the bread for 30-35 minutes, or until the top is golden brown and a wooden skewer inserted into the center comes out clean. If the top is browning too quickly, you can loosely tent it with aluminum foil for the last 10-15 minutes of baking.

8. 8. Cool and Serve: Once baked, remove the loaf from the oven and let it cool in the pan for 10-15 minutes before transferring it to a wire rack to cool completely. Slicing it while still warm is tempting, but a slightly cooled loaf will hold its shape better and the swirls will be more defined.