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

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

3. In a separate medium bowl, whisk together the sourdough discard, milk, large egg, melted unsalted butter, and vanilla extract until well combined. The mixture should be smooth.

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. Overmixing develops gluten, which can result in tough muffins.

5. Gently fold in the finely diced apple pieces until they are evenly distributed throughout the batter.

6. Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full.

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

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