Preheat your oven to 375°F (190°C). Line a muffin tin with paper liners or grease each cup with butter or cooking spray.
In a small bowl, combine the flour, granulated sugar, and ground cinnamon for the crumb topping. Drizzle in the melted butter and mix until the mixture forms coarse crumbs. Set aside.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, salt, ground cinnamon, and ground nutmeg.
In a separate bowl, whisk together the melted butter, milk, egg, and vanilla extract until well combined.
Gently fold the wet ingredients into the dry ingredients, mixing until just combined. Do not overmix; a few lumps are okay. Fold in the diced apples.
Divide the batter evenly among the prepared muffin cups, filling each about two-thirds full.
Generously sprinkle the crumb topping over the muffin batter in each cup, pressing it gently to adhere.
Bake in the preheated oven for 18-20 minutes or until a toothpick inserted into the center of a muffin comes out clean or with a few moist crumbs.
Allow the muffins to cool in the tin for a few minutes before transferring them to a wire rack to cool completely. Once cooled, these Apple Crumb Muffins are ready to be enjoyed!