In a large mixing bowl, whisk together flour, sugar, baking powder, and salt until well combined. Cube cold unsalted butter and cut it into the flour mixture until it resembles coarse crumbs. Stir in the sourdough discard, milk, and vanilla extract until a soft dough forms.

Turn the dough out onto a lightly floured surface and gently knead it 3–4 times without overworking. Shape into a rough rectangle and let it rest for 5 minutes.

Roll the dough into a 10x14-inch rectangle. Spread softened unsalted butter over the surface. Mix brown sugar and ground cinnamon, then sprinkle it over the buttered dough.

Starting from one long edge, roll the dough into a tight log, sealing the edge by pinching it closed. Slice into 8 equal rolls and arrange in a greased baking pan.

Preheat your oven to 400°F (200°C) and allow the rolls to rise slightly while the oven heats. Bake for 15–20 minutes until golden brown.

Whisk together powdered sugar, milk, and vanilla extract until smooth. Drizzle the glaze over the warm rolls before serving.