1. Combine sourdough starter discard, warm milk, melted butter, sugar, and beaten egg in a large bowl and whisk until smooth

2. Whisk flour and salt in a separate bowl, then gradually add to wet mixture until a sticky dough forms

3. Turn dough onto a lightly floured surface and knead for 8-10 minutes until soft, smooth, and elastic

4. Place dough in a greased bowl, cover, and bulk ferment for 4-6 hours or refrigerate overnight after two hours for slow fermentation

5. While dough ferments, mix softened butter, brown sugar, and cinnamon into a smooth filling paste

6. Punch down the dough, roll into a 12×18 inch rectangle about 1/4 inch thick

7. Spread the cinnamon-sugar-butter filling evenly over the dough

8. Roll the dough tightly into a log and seal the seam, then cut into 12 equal rolls

9. Arrange rolls in a greased 9×13-inch baking dish, cover, and proof for 2-4 hours until doubled in size

10. Preheat oven to 375°F (190°C) and brush roll tops with milk or melted butter

11. Bake rolls for 22-28 minutes until golden and an internal temperature of 190°F (88°C) is reached

12. Let rolls cool in the pan for 10 minutes before glazing or serving