Preheat oven to 350°F. Line the bottom of 3 round cake pans with parchment paper and grease the sides. In the bowl of a stand mixer fitted with the paddle attachment cream butter and sugar on medium speed until light and fluffy, 3-4 minutes. Add sour cream and vanilla and mix until combined. Add 3 egg whites and mix until well combined. Add remaining 3 egg whites and mix until well combined. Scrape down sides of bowl as needed.

Whisk together flour, baking powder, cinnamon, and salt in a medium bowl. In a small bowl combine milk and water. With mixer on low speed add half of flour mixture to batter and mix until combined. Add milk mixture to batter and mix until combined. Add remaining flour and mix until smooth, scraping sides of the bowl as necessary. Divide the batter evenly among the 3 cake pans. Bake 23-25 minutes or until a toothpick inserted in the center of the cakes comes out clean. Cool cakes 2-3 minutes in pan and then remove from pans onto a wire rack to cool completely.

In the bowl of a stand mixer fitted with paddle attachment beat butter and shortening on medium speed until smooth. Slowly add in 6 cups powdered sugar and mix until smooth. Add vanilla, cinnamon, and water or milk and mix until smooth. Slowly add remaining 6 cups powdered sugar and mix until smooth, scraping sides of bowl as necessary. There could be frosting left over.

In a small bowl whisk together powdered sugar, brown sugar, cinnamon, and water until smooth. Set aside.

Use a large serrated knife to carefully remove any domes the cake formed when baking. Place the first cake on a serving plate. Spread 2½ Tbsp of glaze over cake layer. Allow it to soak into the cake. Spread 1 cup of frosting onto the top of the cake. Repeat the process with the second layer of the cake, and then add the final layer of cake and frost as desired.