Please read About Buttercream.

This egg white buttercream does not require a cooked syrup. Have the butter and egg whites at room temperature, 68° to 70°F.

Have ready:

Whisk together in a large stainless steel bowl or the bowl of a stand mixer:

Set the bowl in a deep skillet filled with about 1 inch of simmering water. Make sure the water level is at least as high as the depth of the egg whites in the bowl. Beat the whites constantly on low speed with a hand mixer or a whisk until the mixture reaches 160°F on an instant-read thermometer. Do not stop beating while the bowl is in the skillet, or the egg whites will overcook. If you cannot hold the thermometer in the egg whites while continuing to beat, remove the bowl from the skillet just to read the thermometer, then immediately return the bowl to the skillet. Remove the bowl from the skillet and add:

If using a stand mixer, attach the bowl to the mixer and use the whisk attachment to beat on high speed until thick, glossy, and stiff. Continue to beat until the bottom of the bowl is no longer warm, about 10 minutes. Switch to the paddle attachment and beat in the softened butter a couple of tablespoons at a time, waiting until each addition is fully incorporated before adding the next. Beat until smooth. This keeps, refrigerated, for up to 6 days; or freeze for up to 6 months. Soften as for French Buttercream.

We find this buttercream, made with a cooked sugar syrup, to be slightly easier than Swiss meringue buttercream. If the frosting looks broken, don’t panic. Just keep beating until smooth.

Have ready:

Prepare:

When the bowl of the mixer is no longer warm, beat in the softened butter about a tablespoon at a time until the frosting is very smooth and creamy.