Preheat your oven to 350F.
Mix the sour cream and milk in a bowl. No need to make the mix perfect, clumps are fine.
With a whip attachment, beat the butter, sugars and vanilla until you get a pale creamy consistency. Scrape the bowl several times as you do this.
While the mixer is on medium speed, add eggs one at a time. Go back to high speed and whip until you get a pale stiff whipped cream-consistency. Scrape the bowl a few times as you whip.
On low speed, add the milk-sour cream liquid and the cake flour alternating. In other words, add some liquid, then some flour, then some liquid, and then more flour. Do this in 3-4 steps until all the ingredients are in. Add your baking powder during one of the increments. Stop and scrape the bowl several times as you progress.