1. Prepare three 8 inch cake pans with parchment paper circles in the bottom and grease the sides. Preheat oven to 350°F (176°C) .
2. In a large mixing bowl, cream butter and sugar together until light in color and fluffy, about 3-4 minutes. Do not skimp on the creaming time.
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 dry ingredients in a separate bowl, then combine the milk and water in a small measuring cup.
6. Add half of the dry ingredients to the batter and mix until well combined. Add the milk 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. Divide the batter evenly between the cakes pans and bake for about 21-23 minutes, or until a toothpick comes out with a few crumbs.
8. Remove the cakes from the oven and allow to cool for about 2-3 minutes, then remove to cooling racks to cool completely.
9. To make the frosting, beat the butter and shortening together until smooth.
10. Slowly add half of the powdered sugar and mix until smooth.
11. Add the vanilla extract, ground cinnamon and water or milk and mix until smooth.
12. Slowly add remaining powdered sugar and mix until smooth. Set aside.
13. To make the cinnamon sugar glaze, add the powdered sugar, brown sugar and cinnamon to a small bowl and whisk to combine. Add the water and whisk until smooth.
14. To put the cake together, use a large serrated knife to remove the domes from the top of the cakes so that they’re flat. Place the first cake on a serving plate or a cardboard cake round.
15. Spread 2 1/2 tablespoons of cinnamon sugar glaze over the cake layer. Allow it to soak into the cake. Pipe dollops of frosting onto the cake. Try to make sure the dollops are about the same size/height so the next cake doesn’t sit lopsided.
16. Repeat the process with the second layer of cake, glaze and frosting, then add the final layer of cake on top and the final layer of cinnamon glaze. Set cake aside.
17. To make the icing, combine all ingredients except for the powdered sugar in a small sauce pan. Heat on low until sugar is melted, then turn heat to medium to bring to a boil. Boil for 3 minutes, stirring occasionally, then remove from heat and immediately pour into a heat-proof glass bowl.
18. Whisk in powdered sugar and allow to cool slightly and thicken. If you find your icing to be a little thick, just add a touch more water to thin it back out.
19. Drizzle the icing down the sides of the cake while still a little warm. It will be too firm to pour and get a nice drizzle if it cools completely.
20. Use the remaining frosting to pipe dollops around the outside edge of the cake.
21. Fill in the top of the cake – between the dollops – with the remaining icing. Finish off the cake with a sprinkle of ground cinnamon, if desired.

Store the cake at room temperature in an airtight container.