Activate your sourdough discard: If needed, feed your discard 6-8 hours before starting.

Combine wet ingredients: Whisk together sourdough discard, warmed milk, egg, sugar, salt, and melted butter.

Add dry ingredients: In a separate bowl, whisk flour, baking powder, and baking soda. Gradually add to wet ingredients, mixing until a shaggy dough forms.

Knead the dough: Turn out onto a floured surface and knead for 5-7 minutes until smooth and elastic. Add flour if needed.

First rise: Place dough in a lightly oiled bowl, cover, and let rise in a warm place for 1-1.5 hours, or until doubled.

Prepare filling: Cream together softened butter, brown sugar, and cinnamon.

Roll out and fill: Punch down dough, roll into a 12x18 inch rectangle, and spread filling evenly.

Roll, slice, and rise: Roll dough into a log, cut into 12 slices, place in a greased 9x13 inch pan, and let rise for 30-45 minutes.

Bake: Preheat oven to 375°F (190°C). Bake for 20-25 minutes, or until golden brown.

Make icing (optional): Whisk together powdered sugar, milk, and vanilla extract. Add more milk if needed.

Cool and frost: Let cool slightly before frosting (if using). Serve warm.