Activate your sourdough discard: If your discard isn't already active and bubbly, feed it 6-8 hours before starting.

Combine wet ingredients: In a large bowl, whisk together the sourdough discard, warmed milk, egg, sugar, salt, and melted butter until well combined.

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

Knead the dough: Turn the dough out onto a lightly floured surface and knead for 5-7 minutes, until it becomes smooth and elastic. If the dough is too sticky, add a little more flour, a tablespoon at a time.

First rise: Place the dough in a lightly oiled bowl, turning to coat. Cover the bowl with plastic wrap and let rise in a warm place for 1-1.5 hours, or until doubled in size.

Prepare the filling: While the dough is rising, in a medium bowl, cream together the softened butter, brown sugar, and cinnamon until well combined.

Roll out, fill, and roll up the dough: Once the dough has risen, punch it down gently and roll it out into a 12x18 inch rectangle. Spread the cinnamon butter mixture evenly over the dough. Starting from the long side, tightly roll the dough into a log. Cut the log into 12 equal slices.

Place in pan, second rise, and bake: Place the rolls in a greased 9x13 inch baking pan. Cover the pan with plastic wrap and let rise for another 30-45 minutes, or until almost doubled in size. Preheat oven to 375°F (190°C). Bake for 20-25 minutes, or until golden brown.

Make the icing (optional): While the rolls are baking, whisk together the powdered sugar, milk, and vanilla extract until smooth. Add more milk if needed to reach desired consistency.

Cool and frost: Let the rolls cool slightly in the pan before frosting with the icing (if using). Serve warm and enjoy!