In a large mixing bowl, combine your sourdough discard, all-purpose flour, sugar, baking powder, and salt. Stir until a soft dough begins to form, ensuring no dry flour remains.

Add the cold, cubed unsalted butter to the dry mixture, and use a pastry cutter or your fingertips to blend until the mixture resembles coarse crumbs.

Pour in the milk and optional vanilla extract, mixing gently until everything just comes together. Adjust the consistency with additional milk if needed.

Transfer the dough onto a floured surface and knead gently for about 1-2 minutes until smooth.

Using a rolling pin, flatten the dough into a rectangle about ¼ inch thick.

Spread the softened unsalted butter evenly across the dough, followed by a generous sprinkle of brown sugar and ground cinnamon.

Starting from one long edge, tightly roll the dough into a log. Cut this log into 8 equal pieces and arrange in a greased baking dish.

Preheat your oven to 375°F (190°C). Bake for about 20 minutes until golden brown.

While the rolls are baking, whisk together powdered sugar, a splash of milk, and optional vanilla extract until smooth.

Once the rolls have cooled slightly (about 5 minutes), drizzle the prepared glaze over the top.