Preheat the oven to 350 degrees . Grease a 9-by-3-inch round springform pan and a 6-by-3-inch round springform pan.
Using an electric mixer, cream the butter, sugar and orange peel at high speed until fluffy, about 5 minutes, scraping down the sides occasionally. Add the eggs one at a time, beating well after each addition. Mix in the baking powder, baking soda and salt and mix at medium speed, then mix in the vanilla. Add 2 cups flour and 1 cup sour cream and mix at low speed until barely blended. Repeat with the remaining 2 cups flour and 1 cup sour cream.
Spoon the batter evenly into the prepared cake pans, filling them about half full. Bake until the cakes are deep golden and a toothpick inserted into the center comes out clean, about 1 hour and 10 minutes.
Transfer the pans to a rack to cool for 20 minutes, then remove the sides of the pans and let the cakes cool completely.