Makes: Makes about 6 cups
1¼ cups sugar, divided into 1 cup and ¼ cup
⅓ cup water
6 large egg whites, at room temperature
¾ teaspoon cream of tartar
2¼ cups (4½ sticks) unsalted butter, very soft, cut into pieces
Place 1 cup of sugar and water in a small saucepan. 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. Allow sugar mixture to simmer gently as you proceed with egg whites.
Meanwhile, place egg whites in a clean, grease-free mixing bowl and whip until frothy on low speed using the wire-whip attachment of a standing mixer. Add cream of tartar and turn speed to medium-high. When soft peaks form, add ¼ cup sugar gradually. Continue whipping until stiff, glossy peaks form.
Bring the sugar/water mixture to a rapid boil and cook until it reaches 248˚ to 250˚ F. As syrup cooks, check visual cues to assess doneness if you do not have a thermometer: it starts out with a thin consistency and many small bubbles covering the entire surface. As the water evaporates, the mixture will become visibly thicker. Bubbles become larger and pop open more slowly. At this point the syrup definitely looks thickened, but it has not begun to color; this is the firm ball stage - if you drop a bit of the syrup into a glass of cold water it will form into a ball. When you squeeze the ball between your fingertips, it will feel firm and the syrup is ready.
With the mixer running, pour syrup in a thin, steady stream directly over the meringue. Do not pour any on the whip or the sides of the bowl. Whip meringue until cool to the touch; this could take several minutes. With the mixer running, add butter a couple tablespoons at a time. Keep beating until the buttercream is completely smooth and spreadable, somewhere in texture between peanut butter and mayonnaise. Now the buttercream is ready to use. Any flavorings may be added at this point; variations are given below. Refrigerate up to 1 week in an airtight container or freeze up to 1 month. If frozen, defrost in the refrigerator overnight and bring to warm room temperature before re-beating. Always re-beat before using.