01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or grease lightly.
02 - In a large bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
03 - In a separate bowl, whisk melted butter and sugar until combined. Add eggs, vanilla, milk, and yogurt; whisk until smooth.
04 - Fold wet ingredients into dry mixture until just combined, avoiding overmixing.
05 - Gently fold diced apples into the batter.
06 - Divide batter evenly among muffin cups, filling each about 3/4 full.
07 - Mix brown sugar and cinnamon; sprinkle over muffins if using.
08 - Bake 20 to 22 minutes, or until a toothpick inserted in the center comes out clean.
09 - Cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.