Old Time Boiled Icing

I found this cookbook my mom had, and on the cover she had written page 62,  fluffy frosting.  Reading it I remember her making this frosting often, how it would cook and she would look for the thread coming off of the spoon.  

Old time Boiled Icing

Ingredients:

2 cups sugar
1/4 tsp cream of tartar
3/4 cup water
2 egg whites
1 tsp vanilla

Directions:

Mix sugar, cream of tartar and water together in a saucepan.  Boil without stirring, until mixture  spins a 6-inch thread (240 degrees), keeping pan covered for first 3 minutes of cooking to prevent crystals forming on sides of pan.

Beat egg whites stiff.  Pour hot syrup slowly into egg whites, beating constantly at high speed.  Add vanilla.  Continue beating until mixture is fluffy and will hold its shape.  If icing does not set so that it will stay on cake, add 2 or 3 tablespoons powdered sugar, a tablespoon at a time, until mixture is stiff enough.