Make the muffins

1. Preheat the oven to 375°F. Prepare a 12-cup muffin tin with butter
   and flour, or if using a good-quality baking spray, apply just before
   filling.
2. In a medium bowl, sift together the flour, baking powder, salt, and
   cinnamon.
3. In a medium bowl, sift together 1 tbsp flour and ½ tsp cinnamon.
   Toss the diced apples in it to coat.
4. In the bowl of a stand mixer fitted with the paddle attachment,
   beat the butter and sugar until light and fluffy (3 minutes). Add
   the eggs, one at a time, beating well between additions (1 minute
   each). Add the vanilla.
5. Beat in the flour and milk alternately, beginning and ending with
   flour. Scrape down the bowl. Fold in the apples.
6. Fill the prepared muffin tin, each cavity about 2/3 full.
7. Bake until a toothpick or skewer inserted in the middle comes out
   clean (30 minutes). Let cool for 10 minutes before removing it from
   the tin and placing it on a rack.
8. In a small bowl, mix together the granulated sugar and cinnamon.
9. Place the rack of muffins over a rimmed baking sheet to catch any
   drips of butter and excess sugar topping.
10. Brush the top of each muffin with butter and sprinkle with the
    sugar/cinnamon mixture.
11. Let cool completely.