Preheat oven to 375º F. Spray muffin tins with bakers spray or coat well with shortening or butter and flour, making sure to discard any excess flour from the tins after coating.
Sift together 2 cups flour, baking powder, salt, and 2 teaspoons cinnamon in a medium bowl. Set aside.
Mix 2 teaspoons flour with 1/2 teaspoon cinnamon. Toss diced apples with the flour/cinnamon mixture to coat apples in a separate bowl. Set aside.
Cream together butter and sugar until lightened in color, about 3 minutes. Add an egg, one at a time, taking care to fully incorporate before adding the other. Mix in vanilla.
Gently fold in flour mixture, alternating with milk. Stir until just combined. Fold in diced apples and scoop mixture into prepared muffin tins, filling about 2/3 to 3/4 full. Bake until a toothpick or skewer inserted in the middle comes out clean, about 30 minutes.
Prepare topping for muffins while the muffins are baking by melting the butter and allowing to cool slightly. Pour butter into a separate bowl sized easy for dipping tops of muffins. Mix together granulated sugar and cinnamon in a separate bowl and set aside.
Once muffins have baked, remove from the oven and allow to cool slightly in the muffin tin. Then, remove each muffin and brush the butter on with a pastry brush (or dip into the melted butter) and then into the cinnamon sugar mixture. Place onto a plate to finish cooling.
Updated Cinnamon Sugar Topping: The original recipe called for a 1 to 1 mixture of sugar and cinnamon. If you prefer a less cinnamon flavor in your topping, reduce the mixture to 1/4 cup granulated sugar and 1 teaspoon ground cinnamon.