Heat a waffle iron. In a mixing bowl, mix together the cake flour and baking soda and set aside. Separate the egg whites and egg yolks. Whisk together egg yolks, sour cream, milk and vanilla extract. Melt the butter and then mix into the sour cream mixture.
In a different bowl, beat the egg whites with a hand-mixer on medium speed until soft peaks form, about 3 minutes. Add the granulated sugar to the egg whites and continue to beat until stiff peaks form, about 3 to 4 more minutes.
Whisk the sour cream mixture into the flour mixture until just combined. Gently fold in the beaten egg whites until mixture is smooth; do not overmix.
Add about 1/2 cup of the batter to the waffle iron, close and cook for about 3 minutes. Serve waffles with the powdered sugar. Other suggested toppings include fresh berries, whipped cream, jam or syrup.