If you over whip it, it will get full of air and be impossible to spread — you can thin it back down with a little light corn syrup. It will take longer to whip up on days that are humid.
It sets very quickly, so apply it to the cake as soon as it is made.
2 egg whites
1 1/2 cups white sugar
2 teaspoon light corn syrup
1/2 teaspoon Cream of Tartar
1/3 cup cool water
1 dash salt
1 teaspoon vanilla
Put all the ingredients save vanilla in top of a double boiler away from heat. Beat to mix for 30 seconds.
Now, put that top pot into place over the bottom pot, ready with the water in it boiling.
Leave it there for about 7 minutes to cook, beating non-stop.
Then remove the top pot from the heat, add vanilla, and beat for another two minutes.
Makes enough for 2 8-inch cake layers.
Literature & Lore
“1. Cooked Frostings — (a) The so-called boiled frosting — a sugar and water syrup poured and beaten into stiffly beaten egg whites; the White Mountain frosting (the same but with a larger proportion of egg white); and the seven minute frosting, for which all ingredients are put together in the double boiler, and cooked under constant beating.” — Caldwell, Katherine. The National Cooking School Series: Lesson 10. St Petersburg, Florida: The Independent. Tuesday, 12 February 1935