Preheat the oven to 450°F. Grease and flour a 12-cup muffin pan, or line with papers and grease the insides of the papers.
Gently mix in the buttermilk, going slowly so it won't curdle with the butter-sugar mixture.
Add the combined flours, baking powder, baking soda, salt, and cinnamon and stir until just mixed. Fold in the chopped apples.
Divide the batter evenly among the prepared muffin cups, sprinkling the remaining 1/2 cup brown sugar on top. Use it all!
Bake the muffins for 15 minutes, and then lower the temperature to 400 degrees and bake for about 5-10 minutes more, or until a toothpick inserted into the center of a muffin comes out clean.
Remove the muffins from the oven, cool them for 5 minutes in the pan, then turn them out onto a rack to finish cooling completely...or as long as you can wait before gobbling up.