01 - Set oven to 375°F. Line a 12-cup muffin tin with paper liners or grease lightly.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
03 - In a large bowl, whisk melted butter with sugar until well blended. Incorporate eggs one at a time, mixing after each addition, then stir in milk and vanilla.
04 - Gently fold dry ingredients into wet mixture until just combined, avoiding overmixing.
05 - Fold diced apples into batter until evenly distributed.
06 - Divide batter evenly into muffin cups, filling each about three-quarters full.
07 - In a small bowl, combine flour, brown sugar, cinnamon, and salt. Cut in cold butter using a pastry blender or fingers until mixture forms coarse crumbs.
08 - Sprinkle streusel evenly over each muffin.
09 - Bake for 20 to 22 minutes, or until a toothpick inserted in the center comes out clean.
10 - Allow muffins to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.