Preheat the oven to 350 degrees Fahrenheit. Butter and line two 8 x 3 inch baking pans or one 12 x 3 pan with parchment. Sift together the flour, baking powder, and salt. Cream the butter in a large bowl with an electric mixer until fluffy and light color, about 2 minutes on medium speed. Add the sugar and continue to mix until fluffy and light. Add the egg yolks. one at a time, being sure each is well incorporated before adding the next one. Add the vanilla. Reduce the mixture speed to low and add the dry ingredients alternately with the sour cream, beginning and ending with the flour. Be sure the mixture is completley blended. In a separate bowl, with clean beaters, beat the egg whites to soft peaks. Gently fold the whipped egg whites into the batter with a rubber spatula. Pour the batter into the prepared pans and smooth with a rubber spatula. Bake in preheated oven, 60 minutes for the 12-inch square pan or 45 to 50 minutes for the 8-inch pan. The top of the cake should be nicely browned. Test for doneness with a skewer or a toothpick. The tester should come out dry and clean.