This is such a great cake. For really stellar flavor I use Navan, which is a cognac infused with Madagascar vanilla. http://www.navanworld.com/navan.htm
It has almost the exact same amount of alcohol as vanilla extract, so while the 1/4c. in the recipe may seem like a lot, the alcohol bakes away and you're left with a simply awesome taste.

Ingredients:

• 4 egg yolks
  
• 2/3 c.sour cream
  
• 2 T vanilla extract 
  
• 1/4 c. Vanilla Cognac
  
• 2 c. cake flour (sifted, then measured)
  
• 1 c. sugar
  
• 1/2 t. baking powder
  
• 1/2 t. baking soda
  
• 1/2 t. salt
  
• 6 oz. unsalted butter at room temperature
  

Instructions:

1. In a small bowl whisk yolks, vanilla, cognac and 1/3 cup sour cream together. 
   
2. In your mixer bowl, with the paddle attachment, place all of your dry ingredients, and mix briefly to combine. 
   
3. Add butter and the remaining sour cream, mix on low speed until combined. Then adjust your mixer to medium speed and blend for about 2 minutes. 
   
4. Scrape down the sides, and add your egg yolk mixture in three batches, beating for 30 seconds and scraping the bowl after each addition. 
   
5. Bake in a 9 inch springform pan lined with parchment at 300 for about 35 minutes. The cake should be flat (no domed top) and just under 2 inches tall.
   
6. Torte, fill, and frost as desired.