Following the ingredient amounts for French Meringue, place the egg whites, vanilla, if using, cream of tartar, and sugar in a large heatproof bowl or the bowl of a stand mixer. Set the bowl in a skillet of barely simmering water and whisk until the whites are warm, not hot, to the touch (110° to 115°F). Remove the bowl from the skillet and beat on high speed (with the whisk if using a stand mixer) until the whites are stiff.