In a heatproof bowl of a stand mixer, whisk egg whites, sugar, water and cream of tartar until blended. Place over simmering water in a large saucepan over medium heat. Whisking constantly, heat mixture until a thermometer reads 160°, 3-5 minutes.
Remove from heat; add vanilla. With the whisk attachment of a stand mixer, beat on high speed until stiff glossy peaks form, about 7 minutes. Use immediately. Store frosted cake, uncovered, in refrigerator.