01 - Preheat oven to 375°F. Line a 12-cup muffin tin with paper liners or lightly grease.
02 - Whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg in a medium bowl.
03 - In a large bowl, whisk melted butter and granulated 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 without overmixing.
05 - Fold in diced apples evenly through the batter.
06 - Divide batter evenly into muffin cups, filling each about three-quarters full.
07 - Mix flour, brown sugar, cinnamon, and salt in a small bowl. Cut in cold butter with pastry blender or fingers until coarse crumbs form.
08 - Sprinkle streusel mixture evenly over each muffin.
09 - Bake for 20 to 22 minutes or until a toothpick inserted in the center comes out clean.
10 - Cool muffins in the tin for 5 minutes, then transfer to a wire rack to cool completely.