Whisk together the starter, buttermilk, egg, and melted butter until smooth. Add the flour, sugar, and salt, stir to a shaggy dough, then knead on a floured surface for 8 to 10 minutes until smooth and elastic.

Place dough in a greased bowl, cover, and let rise at room temperature for 4 to 5 hours, performing three sets of stretch and folds during the first 90 minutes.

Punch down dough, roll into a 9x13-inch rectangle, spread the 3 tbsp soft butter, and sprinkle with the brown sugar and cinnamon mixture. Roll tightly from a short side, seal the seam, and place in a buttered 9x5-inch loaf pan.

Cover and proof until dough rises 1 inch above the pan, 2 to 3 hours. Preheat oven to 375°F (190°C) during the last 20 minutes. Bake for 45 to 50 minutes until deep golden and internal temp reaches 190°F to 200°F (88°C to 93°C).

Cool in pan 10 minutes, then turn onto a rack and cool completely. Slice with a serrated knife and serve.