Preheat oven to 350 degrees. Prepare round 9-inch cake pan by spraying with nonstick spray. Set aside.
Add sugar and butter to a mixing bowl fit for a stand mixer. Using the paddle attachment, cream together until light and fluffy, about 3 minutes. Add sour cream and mix until combined. Add 1 egg and mix until combined. Add vanilla and mix until combined.
In a separate medium bowl, mix flour, baking powder, and salt. In a small bowl, mix water and milk together. Add half of dry ingredients to batter and mix until combined. Add milk mixture and mix until combined. Add remaining dry ingredients and mix until combined. Add batter to prepared dish and spread into one even layer. Set aside while we make the chocolate cake batter. Both will bake at the same time.
For the chocolate cake, add the flour, sugar, cocoa powder, baking soda, and salt to a large bowl. Whisk together until combined. In a separate smaller bowl, whisk eggs, oil, and vanilla together until combined. Add the wet ingredients to the dry and fold a few times but not mixing completely. Add the hot water and mix together until all combined. Prepare square 9x9 square pan by spraying with nonstick spray. Add in chocolate cake batter and spread into one even layer.
Bake cakes side by side in preheated oven for about 18-20 minutes, or until toothpick comes out clean when inserted in the center. Vanilla cake may be ready before the chocolate cake. Let cakes cool completely before moving on.