1. Preheat oven to 350°F (176°C) and prepare a cupcake pan with cupcake liners.
2. In a large mixing bowl, cream butter and sugar together until light in color and fluffy, about 3-4 minutes.
3. Add sour cream and vanilla extract and mix until well combined.
4. Add egg whites in two batches, mixing until well combined after each. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
5. Combine the flour, baking powder and salt in a separate bowl, set aside. Combine the apple butter and milk in a small measuring cup.
6. Add half of the dry ingredients to the batter and mix until well combined. Add the apple butter mixture and mix until well combined. Add remaining dry ingredients and mix until well combined. Scrape down the sides of the bowl as needed to be sure all ingredients are well incorporated.
7. In a small bowl, combine the cinnamon and sugar.
8. To fill the cupcake liners, add a little less than a tablespoon of batter to each liner. Sprinkle some cinnamon sugar over the batter, covering it. Add a little more batter to the liner, then another layer of cinnamon sugar. Finish them off with a little more batter, so the liners are filled about halfway, or just a bit more.
9. Bake for 17-19 minutes, or until a toothpick inserted comes out with a few crumbs. Remove from the oven and set aside on a cooling rack to cool.
10. To make the frosting, combine the butter and shortening and mix until smooth.
11. Add about half of the powdered sugar and the apple butter and mix until well combined.
12. Add the remaining powdered sugar and mix until smooth.
13. Pipe the frosting onto the cooled cupcakes. I used Ateco tip 844.
14. Sprinkle on some of the remaining cinnamon sugar from the cupcake filling, then top the cupcakes with some apple chips or mini snickerdoodles.