Whisk water, cream of tartar, sugar, egg whites and corn syrup 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 egg 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 whiles 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 5 minutes. Remove the bowl from the skillet and add the vanilla. Beat on high speed for 2 to 3 more minutes to cool. Stir in the nuts or coconut if desired.
Seven Minute Lemon Frosting
Prepare frosting , above, substituting 2 to 3 tablespoons fresh lemon juice for 2 to 3 tablespoons of the water and adding 1/4 teaspoon grated lemon zest.
Seven Minute Orange Frosting
Prepare frosting , above, substituting 1/4 cup fresh orange juice for all of the water and adding 1/2 teaspoon grated orange zest.
Seven Minute Penuche or Brown Sugar Frosting
Prepare frosting , above, substituting 1 1/3 Cups packed light or dark brown sugar for the sugar. Add 1/2 teaspoon maple, walnut, or pecan extract with the vanilla if desired.