Preheat the oven to 350º F.
Using butter, grease the bottom and sides of an 6 inch round cake pan or springform pan and line the bottom with a round piece of parchment paper. To ensure even baking, place a bake even strip around the pan. You can make your own homemade bake even strip by cutting a towel or shirt to fit the size of your pan. Get the fabric really wet, then squeeze out the dripping water but do not squeeze it too dry. Secure the fabric around the pan with a safety pin.
In a mixing bowl, combine the butter and sugar, beat using an electric mixer until fluffy. Add in the egg whites, beat again.
Add in the sour cream and vanilla extract, mix until smooth.
In a smaller bowl, combine the flour, baking powder and salt, gently mix with a spoon. Gradually add half of the flour mixture to the wet ingredients, do not dump it in, rather take spoonfuls of the flour mixture and gently shake it over the wet ingredients, as if you were sifting in the flour. Fold in the mixture until no flour remains. Repeat with the other half of the flour, folding it in and scraping the sides and bottom of the bowl to incorporate everything. Scrape the batter into the prepared cake pan, using a spatula to evenly spread it out.
Bake for 23-25 minutes, or until a toothpick inserted in the center comes out clean.
Once the cake is done, allow it to cool in the pan on a cooling rack for 10 minutes. After 10 minutes, carefully remove the cake from the pan. If using a springform pan, remove the sides and bottom. Allow the cake to cool completely on a cooling rack. Once cool, remove the parchment paper round from the bottom of the cake. If you need to level the top of your cake, do so now using either a cake lever or knife. Make sure the cake is completely cooled, then wrap the cake in plastic wrap and place it in the refrigerator. This cake is good for up to one week like this.