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.

TO MAKE THE BUTTERCREAM:

9. In a large mixer bowl, beat the butter and shortening together until smooth.
10. Slowly add half of the powdered sugar and mix until smooth.
11. Add the rose water, vanilla extract and 4-5 tablespoons of water or milk and mix until smooth.
12. Slowly add the remaining powdered sugar and mix until smooth. Add additional water or milk, as needed.
13. Color the frosting with the ivory icing color. You will add the pink color later.

TO ASSEMBLE THE CAKE:

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 about 1 cup of rose water frosting evenly on top of the cake.
16. Add the second layer of cake and another cup of rose water frosting.
17. Top the cake with the remaining layer and frost the outside of the cake. Refer to my tutorial for frosting a smooth cake, if needed, or watch the video above.
18. Color the remaining frosting with a little bit of pink icing color.
19. To create the light watercolor effect, use an offset spatula to add a thin layer of pink frosting around the cake. I added a fairly full layer and then removed as much as I wanted. You could also just add the pink frosting in certain spots and smooth it from there. Again, watch the video above for an example.
20. Use the remaining pink frosting to pipe rosettes around the top edge of the cake. I used Ateco tip 844, but Wilton 1M or 2D would work as well.
21. Sprinkle the cake with some sparkling sugar sprinkles, if desired.