Preheat oven to 350 degrees. Grease a 9-inch pan, dust it with flour, and line the bottom with waxed paper (note: I used parchment paper). In a bowl, whisk together the yolks, ¼ of the sour cream, and the vanilla. In a large, separate bowl, mix the flour, sugar, baking powder, baking soda and salt; which vigorously to combine.
Add the butter and remaining sour cream to the flour mixture, and mix well until the flour is completely moistened. Add the egg mixture to the flour mixture in three separate additions, mixing between each addition. Pour into prepared cake pan.
Bake in the middle of the oven until a toothpick inserted into the center comes out clean, usually about 35-40 minutes. Start checking at 25 minutes, since oven temperatures and ingredient characteristics vary, and it might be done quicker. Cool 10 minutes, then take out of the pan and cool completely on a wire rack.
To frost, cut laterally in half and frost both sections, then stack, smooth the sides and refrigerate to set.