Heat the oven to 425°F (220°C). Line a 12-cup muffin pan with paper liners or grease it well.
Make the crumb: in a small bowl, stir the flour, brown sugar and cinnamon, then pour in the melted butter and stir with a fork until it forms clumps. Set aside.
In a medium bowl, whisk the flour, baking powder, baking soda, salt, cinnamon and nutmeg. Take out 1 tablespoon and toss it with the diced apples.
In a large bowl, whisk the melted butter with both sugars. Whisk in the eggs one at a time, then the buttermilk and vanilla.
Add the dry ingredients and fold with a spatula until a few streaks of flour remain. Add the apples and fold just until no dry flour is visible.
Divide the batter between the cups, filling them right to the top. Press the crumb lightly onto each one.
Bake for 5 minutes at 425°F (220°C), then, without opening the door, lower the oven to 350°F (175°C) and bake 15 to 18 minutes more, until a toothpick in the center comes out clean.
Cool in the pan for 5 minutes, then move the muffins to a wire rack.