Preheat oven to 350°F (175°C) and line a muffin pan with 12 paper liners.
In a large bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
In another bowl, whisk melted butter, brown sugar, granulated sugar, eggs, and vanilla until smooth. Stir in buttermilk.
Gently fold dry ingredients into wet until just combined. Do not overmix.
Add diced apples and mix lightly.
Spoon batter into muffin cups, filling about ¾ full.
If using, sprinkle crumb topping over each muffin.
Bake for 20–25 minutes or until a toothpick inserted comes out clean.
Let cool for 5 minutes, then transfer to a wire rack.