Place 2/3 cup sugar and the water in a small pot. Stir to wet sugar. Bring to a boil over medium-high heat, swirling pan occasionally.
Dip pastry brush in cold water and wash down sugar crystals from the sides of the pot once or twice. Turn down heat to simmer gently.
Place the egg whites in a clean, grease-free mixing bowl. Using balloon whip attachment to standing mixer, whip until frothy on low speed.
Add cream of tartar and turn speed up to medium high.
When soft peaks form, gradually add remaining 3 tablespoons sugar. Continue whipping until stiff, glossy peaks form. (This is the meringue part of the buttercream, and the frosting may be used at this point for a low-fat, marshmallow-like frosting; it must be used immediately.)
Bring the sugar-water mixture to a rapid boil and cook until it reaches 248° to 250°. As syrup cooks, look for visual clues to assess temperature. At first, it will be thin with many small bubbles over the surface. The water will begin to evaporate and the mixture will become thicker. The bubbles get larger and sticky and pop more slowly. At this point, the syrup looks thickened, but it has not begun to color. If you drop a bit of the syrup into a glass of cold water, it will form into a ball. This is the firm-ball stage, and it means the syrup is ready.
Pour a thin, steady stream over meringue, without pouring any on the rotating whip or the sides of the bowl.
Whip meringue on high speed until cool. When the bowl is no longer warm, stop the machine and touch the surface of the meringue with your finger to check that it is cooled.
With mixer on medium speed, add the butter a couple of tablespoons at a time. Keep beating until the buttercream is completely smooth.
Your buttercream is ready to use. Any flavorings may be added at this point; variations are given in individual recipes. (Reheat after storing.)