1. Preheat your oven to 375 F (190 C). Line a 12-cup muffin tin with paper liners or grease it thoroughly with butter or non-stick spray.

2. In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, 1/2 teaspoon ground cinnamon, and salt. Make sure there are no lumps and the ingredients are well distributed.

3. In a separate medium bowl, whisk together the sourdough discard, milk, melted butter, large egg, and vanilla extract until everything is smoothly combined.

4. Pour the wet ingredient mixture into the dry ingredient mixture. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix; a few lumps are perfectly fine and will result in a more tender muffin.

5. Divide the batter evenly among the 12 prepared muffin cups. In a small bowl, mix the 2 tablespoons of brown sugar with an extra pinch of ground cinnamon. Sprinkle this cinnamon-sugar mixture generously over the top of each muffin.

6. Place the muffin tin in the preheated oven and bake for 18-22 minutes, or until the tops are golden brown and a wooden skewer or toothpick inserted into the center of a muffin comes out clean.

7. Once baked, remove the muffin tin from the oven and let the muffins cool in the tin for 5 minutes. Then, transfer them to a wire rack to cool completely. They are best enjoyed warm, but also delicious at room temperature.