Use this for the Lady and Lord Baltimore cakes (ie white round layers or yellow square layers). Makes enough to fill and frost two 8- or 9-inch cake layers. Included is variation for Pink version. From Family Circle All-Time Baking Favorites.
Combine sugar, corn syrup, water, and salt in a small saucepan; cover. Heat to boiling; uncover; boil gently, until mixture registers 242 degrees F on a candy thermometer, or until a small amount of the hot syrup falls, thread-like, from spoon.
While syrup cooks, beat egg whites with cream of tartar in a large bowl until stiff peaks form when beaters are removed. Pour hot syrup onto egg whites in a very thin stream, beating all the time at high speed, until frosting is stiff and glossy. Beat in the 1/2 teaspoon vanilla.
Pink Mountain Cream Frosting:
Follow basic recipe for White Mountain Cream Frosting, using 1/2 cup maraschino cherry liquid in place of water; add a few drops of red food coloring together with the 1/2 teaspoon vanilla to tint frosting a delicate pink. Add more red food coloring if you prefer a deeper shade.