This recipe requires the simultaneous preparation of a sugar syrup and the beating of eggs or egg yolks. If the syrup heats up too quickly before the eggs are whipped, take it off the heat to delay the cooking. The eggs or yolks may also be whipped until thick before beginning the sugar syrup. In that case, turn the mixer back on high speed when the syrup is almost ready. Have all the ingredients at room temperature, 68° to 70°F.

Combine in a medium heavy saucepan and cook, stirring, over medium heat until the mixture begins to simmer:

Stop stirring, cover, and simmer for 2 minutes to dissolve the sugar. Uncover and wash any sugar crystals from the sides of the pan with a wet pastry brush. Cook, uncovered, until the syrup reaches 235° to 240°F (soft-ball stage) on a candy thermometer.

Meanwhile, in a large bowl, or in a stand mixer with the whisk attachment, beat on high speed until thick and pale yellow:

Just before the syrup is ready, begin beating the eggs again on medium speed. Beating constantly, pour the hot syrup in a thin steady stream into the eggs, avoiding the beaters or whisk. Beat the hot mixture until the bottom of the bowl, when touched, no longer feels warm. Beat in 1 tablespoon at a time:

beating until the buttercream is smooth and spreadable. If the mixture looks curdled at any time, simply continue beating until smooth. If the butter is added too quickly or the ambient temperature is very warm, the mixture may become soupy; refrigerate it for 20 to 30 minutes, then resume beating.

This keeps, refrigerated, for up to 6 days; or freeze for up to 6 months. To soften chilled or frozen buttercream, break it into chunks with a fork. Place it in a large bowl or the bowl of a stand mixer and allow it to come to room temperature. Then beat until smooth and creamy.