Get ready by putting the egg whites and vanilla in a large mixing bowl. Set aside.
In a small saucepan, combine the water, sugar, and cream of tartar over high heat. Bring to a boil for 2-3 minutes, counting once the mixture begins to bubble on the edges. Remove from heat.
On medium-high speed with either a hand or stand mixer, beat the egg whites and vanilla until foamy approximately 1 minute.
Keep the mixer on and with a thin, steady stream, sloooowly pour the sugar water mixture into the egg whites. Continue to beat on medium-high until fluffy and stiff peaks form. This will take around 5 minutes and the frosting will be bright, white and glossy. Try and resist tasting it. Or you'll eat the whole thing. And your cake will be lonely.