1. Preheat your oven to 375 F (190 C). Line a 12-cup muffin tin with paper liners or grease it well to prevent sticking.

2. In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt until evenly distributed.

3. In a separate medium bowl, whisk together the sourdough starter discard, egg, milk, melted butter, and vanilla extract until well combined and smooth.

4. Pour the wet ingredient mixture into the dry ingredient mixture. Stir gently with a spatula until just combined. Do not overmix.

5. In a small bowl, combine the brown sugar, ground cinnamon, and 2 tablespoons melted butter for the swirl. Spoon half of the muffin batter into the prepared muffin cups. Dollop about half of the cinnamon swirl mixture over the batter in each cup, then top with the remaining muffin batter. Spoon the rest of the cinnamon swirl mixture on top of the batter in each cup, using a skewer or toothpick to gently swirl it into the batter.

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

7. While the muffins are cooling slightly in the tin, whisk together the powdered sugar, 2-3 tablespoons milk, and ½ teaspoon vanilla extract in a small bowl until you have a smooth, pourable glaze.

8. Once the muffins have cooled for about 5 minutes in the tin, transfer them to a wire rack. Drizzle the glaze generously over the warm muffins. Allow the glaze to set for a few minutes before serving. These muffins are best enjoyed warm!