1. Preheat oven to 375°F (190°C) and line a 12-cup muffin tin with paper liners or grease lightly.

2. In a large bowl whisk together 2 cups flour, baking powder, baking soda, salt, and 2 teaspoons cinnamon.

3. In another bowl whisk melted butter, brown sugar, and eggs until smooth, then stir in milk, vanilla extract, and sourdough discard.

4. Pour wet ingredients into dry ingredients and fold gently until just combined, leaving small lumps.

5. Fold in chopped apples until evenly distributed in the batter.

6. If desired, let the batter rest for 20–30 minutes for deeper flavor.

7. In a small bowl combine flour, brown sugar, cinnamon, and cubed butter, then pinch with fingers or use a pastry cutter until coarse crumbs form.

8. Spoon batter into muffin cups and sprinkle streusel evenly on top, pressing lightly to adhere.

9. Bake for 20–24 minutes until a toothpick inserted in the center comes out clean and tops spring back lightly.

10. Let muffins cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.

11. Serve warm or store in an airtight container at room temperature for up to two days.