Preheat the oven to 350 degrees F. Line 12 muffin cups with paper liners.
Sift the cake flour, salt and baking powder together and set aside.
In a microwave-safe measuring cup, combine milk and butter. Microwave one minute or long enough to scald milk and melt butter. You may also do this over the stove.
Beat the egg whites in a large bowl until foamy using an electric mixer. Add cream of tartar and continue beating on high until whites form stiff peaks. Set aside.
In a mixing bowl, combine the hot milk/butter mixture with the sugar and beat on low speed. Add the flour mixture and the vanilla and beat until smooth.
Gently fold in part of the egg whites to lighten the batter, then fold in all the rest. Divide evenly among the cupcake containers filling about ¾ of the way full.
Bake on center rack for 20 minutes.
To make the icing, combine all ingredients except vanilla in a metal bowl set over a pan of boiling water. You want it about 2 inches from the water, but mine is almost always closer and I never have a problem. If you have one, you can just use a double boiler.
Beat on high speed of an electric mixer for about 7 minutes or until mixture is thick and spreadable and peaks have formed. Remove from heat and beat in the vanilla.