5 tablespoons water
1/4 teaspoon cream of tartar
1 1/3 cups sugar
2 large eggs whites at room temperature
1 tablespoon light corn syrup
1 teaspoon vanilla
1/2 to 1 cup chopped nuts or shredded sweetened dried coconut (optional)
Whisk first five ingredients together in a large, stainless-steel bowl. Set the bowl in a wide, deep skillet filled with about 1 inch of simmering water. Make sure the water level is at least as high as the depth of the egg whites in the bowl. Beat the whites on low speed until the mixture reaches 140 degrees F on an instant-read thermometer. Do not stop beating while the bowl is in the skillet, or the egg whites will be overcooked. If you cannot hold the thermometer stem in the egg whites while continuing to beat, remove the bowl from the skillet just to read the thermometer, then return the bowl to the skillet.
Beat on high speed for exactly five minutes. Remove the bowl from the skillet and add vanilla, beating on high speed for two to three more minutes to cool. Stir in coconut or nuts, if you are using them.
Use this frosting the day it is made.