Preheat the oven to 375°F (190°C).
Line a 12-cup muffin pan with paper liners or lightly grease the muffin cups.
Peel, core, and finely dice the apples. Set aside.
In a medium bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
In a large bowl, whisk together melted butter, brown sugar, and granulated sugar.
Add the eggs one at a time, whisking after each addition.
Stir in vanilla extract.
Add the buttermilk and mix until combined.
Gradually add the dry ingredients to the wet ingredients.
Gently stir until just combined. Do not overmix.
Fold in the diced fresh apples.
Divide the batter evenly among the 12 muffin cups, filling each nearly to the top.
Mix the topping sugar and cinnamon together.
Sprinkle the cinnamon sugar evenly over the muffins.
Bake for 18–23 minutes, or until the tops are golden and a toothpick inserted into the muffin portion comes out clean or with a few moist crumbs.
Allow the muffins to cool in the pan for about 5 minutes.
Transfer to a wire rack and cool slightly before serving.