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

2. In a large mixing bowl, whisk together the all-purpose flour, 1/2 cup granulated sugar, baking powder, baking soda, and salt. Make sure there are no lumps and everything is evenly distributed.

3. In a separate medium bowl, whisk the large egg until slightly frothy. Then, add the milk, sourdough discard, melted butter, and vanilla extract. Whisk until all the wet ingredients are well 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 lead to tender muffins.

5. Divide the batter evenly among the 12 prepared muffin cups. Each cup should be about two-thirds full.

6. In a small bowl, combine the remaining 2 tablespoons of granulated sugar and 1 teaspoon of ground cinnamon. Sprinkle this mixture generously over the top of each muffin.

7. Bake for 20-25 minutes, or until the muffins are golden brown on top and a wooden skewer or toothpick inserted into the center of a muffin comes out clean.

8. Let the muffins cool in the pan for 5 minutes before transferring them to a wire rack to cool completely. Enjoy them warm or at room temperature.