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

2. In a large bowl, whisk together flour, baking powder, salt, cinnamon, nutmeg, granulated sugar, and brown sugar.

3. In a separate bowl, whisk melted butter, eggs, milk or buttermilk, and vanilla extract until smooth.

4. Pour wet ingredients into dry ingredients and gently fold with a spatula until just combined.

5. Fold diced apples into the batter until evenly distributed.

6. Divide batter among muffin cups, filling each about three-quarters full.

7. Bake for 18–22 minutes or until a toothpick comes out clean and tops spring back lightly.

8. While muffins cool slightly, whisk powdered sugar, milk, and cinnamon in a bowl until smooth for the glaze.

9. Drizzle glaze over warm muffins and let set for a few minutes before serving.

10. Store cooled muffins in an airtight container at room temperature for up to 3 days or freeze for up to 2 months.