I adapted this recipe from Smitten Kitchen's version which can be found here. I found myself out of AP flour (! I know - it was a very upsetting moment, but, to defend myself, it was due to a slightly intoxicated chocolate chip cookie baking moment where I used up the flour - at least it was for a good cause).
To make the muffins:
Combine 2 cups whole wheat flour, 1 teaspoon each baking soda and powder, 1/4 teaspoon salt, 1 tablespoon cinnamon, 1 teaspoon nutmeg and set aside. In a separate bowl cream 1 stick unsalted butter and 1/2 cup each of granulated sugar and dark brown sugar. Add 1 egg to the butter mixture and stir to combine. Then, gently fold in 1 cup buttermilk. Add the dry ingredients to the wet mixture and stir to combine. To finish the batter fold in the apple (I used Honeycrisp but choose your favorite - Granny Smith, Pink Lady, whatever you please - peeled them and chopped them into little chunks). Spoon the batter into greased muffin tins and top with a little bit of brown sugar. Bake at 400 degrees for 15 - 20 minutes or until a toothpick comes out clean.