Prepare the muffin batter: Preheat your oven to 350°F (175°C) and line a 12-cup muffin tin with paper liners. In a large bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, and salt. In a separate bowl, beat together the eggs, granulated sugar, melted butter, and vanilla extract until smooth. Add the milk and stir until combined. Gradually add the dry ingredients to the wet ingredients, mixing until just combined. Gently fold in the diced apples.

1. Make the crumb topping: In a small bowl, combine the flour, granulated sugar, brown sugar, cinnamon, and salt. Add the cold cubed butter and use a pastry cutter or your fingers to mix it until the mixture resembles coarse crumbs.
2. Assemble the muffins: Divide the muffin batter evenly among the 12 muffin cups, filling each about 3/4 full. Sprinkle a generous amount of the crumb topping over the batter in each muffin cup.
3. Bake the muffins: Bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean and the tops are golden brown. Let the muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely.
4. Serve: Enjoy the muffins warm, or store them in an airtight container for later.