01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or lightly grease.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
03 - In a large bowl, whisk melted butter and sugar until combined. Add eggs one at a time, whisking after each. Stir in milk and vanilla extract.
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, mix flour, brown sugar, cinnamon, and salt. Cut in cold butter with a pastry blender or fingers until crumbly.
08 - Sprinkle streusel evenly over each muffin.
09 - Bake for 20 to 22 minutes until a toothpick inserted in the center comes out clean.
10 - Let muffins cool in the tin for 5 minutes before transferring to a wire rack to cool completely.