Preheat the oven to 375F. Grease and flour or line 16 standard-sized muffin tin cups. Set aside.
In a large bowl, beat butter, granulated sugar, and 1/4 cup of the brown sugar together until fluffy and well-combined, about 2-3 minutes on medium.
Add the egg and mix until combined.
Scrape down the sides of the bowl.
Mix in the buttermilk until combined.
Add the flour, baking powder, baking soda, salt, and spices. Stir in just until incorporated. Don't overmix.
Stir in the chopped apples.
Divide the batter evenly among the prepared muffin cups.
Sprinkle the remaining 1/2 cup brown sugar over the batter.
Bake the muffins for 20-30 minutes, or until a toothpick inserted into the center of a muffin comes out clean.
Let the muffins cool in the pan for 5 minutes.
Remove muffins to a wire cooling rack to finish cooling.
Notes
Butter: I use salted butter in this recipe. If you're using unsalted butter, increase the salt to 1/2 teaspoon.
Brown sugar: Light brown sugar or dark brown sugar will work in this recipe.
Buttermilk: Regular or low-fat buttermilk is ok. If you don't have buttermilk, you can make a homemade buttermilk substitute.
Flour: To properly measure the flour, weigh it. To get the weights, click on "metric" immediately above the ingredients. You can also sift or stir the flour to break it up. Then lightly spoon into the measuring cup and level. Avoid packing the flour as that will give you dry, dense, or heavy muffins.
Spices: You can substitute 2 teaspoons of apple pie spice for the spices.
Apples: Any baking apple will work in this recipe. Some of the apples that work well are Granny Smith, Jonagold, or Honeycrisp.