In a small bowl, combine sugar and seeds from the vanilla bean. (For those of you who are new to using vanilla beans, check out this video to learn how to get the seeds out of the bean.)
Using the back of a spoon, move around the bowl and apply pressure to break up any clumps of seeds and to better infuse the vanilla flavor into the sugar. Set aside.
In a medium-sized mixing bowl or bowl of a stand mixer, mix together cake flour, baking powder, baking soda, and salt.
Add the vanilla bean sugar and mix until well combined.
Add butter and mix on medium-low speed for three minutes. Because there is so little butter, you'll end up with a very fine crumb texture.
In a small mixing bowl, whisk together eggs, sour cream, oil, and vanilla extract until smooth.
Add the egg mixture to the flour mixture and beat on medium speed until just combined. Be sure to scrape down the sides of the mixture as needed to make sure that everything is incorporated into the batter.
Slowly add milk and mix on low speed until just combined. The batter will be liquid. (Don't worry, you didn't do anything wrong. It's supposed to be that way.)
Divide the batter evenly between two 8" round cake pans.
Bake for 18 minutes and then test to see if they are done. They are done when a toothpick comes out without wet batter stuck to it. The cakes should appear white with specks of vanilla bean. They should not turn a golden brown. If they are not done, test again in two minutes. If they are still not done, test again in another two minutes.
When the cakes are done, place them on a cooling rack (or just on your counter if you don't own a cooling rack) to cool to room temperature. Then, refrigerate before frosting.