It won’t store long, more than a day or two, or it will separate like meringue.
At one point, you could buy mixes for it.
1 1/2 cups brown sugar
1/2 cup water
1/2 teaspoon of salt
3 egg whites, stiffly beaten
1 teaspoon of vanilla
Combine sugar, corn syrup and boiling water in a saucepan. Over low heat, cooking the mixture. Stir frequently. Sugar should dissolve and the mixture should boil. Continue cooking until firm ball stage is reached.
Pour this syrup in a fine, slow stream into egg white, beating constantly as you do. Add vanilla. Beat until stiff again.
Spread on cake.