Preheat the oven to 350°F. Line muffin pans with paper liners.
In a medium bowl, whisk together the flour, baking powder, cinnamon, and salt. In a large bowl, use an electric mixer to beat together the egg and sugar until thick and well-blended. Gradually mix in the coconut oil until incorporated, and then blend in the sour cream. Slowly mix half of the flour mixture into the wet ingredients, blend in milk, and then stir in remaining flour mixture until just combined. (Add a splash more milk if batter is too thick, but be careful not to add too much!) Gently fold the diced apples into the batter.
Evenly divide batter between 12 to 15 muffin cups. Bake for 20 to 25 minutes or until muffins are light golden brown and a toothpick inserted in the center comes out clean. Allow muffins to cool in pans for a couple of minutes before removing them to a cooling rack.
Once muffins are cool enough to handle (or you may wait until they're completely cool), pour melted butter into a bowl. Mix sugar and cinnamon together in another bowl. Dip the top of each muffin in butter and then press it into the cinnamon sugar mixture until generously coated.
If you can't find whole wheat pastry flour, you may substitute 1 cup all-purpose white flour plus 1 cup whole wheat flour. Or you may use all white flour, if you wish.
I leave the skin on my apples and dice them pretty finely. You may cut them in bigger chunks if you prefer. Any apples will work in these muffins, but some varieties will hold their shape better once baked.
If desired, you may substitute melted butter for the coconut oil.