1. In the bowl of an electric mixer fitted with the paddle attachment, beat butter until smooth.

2. In the heatproof bowl of an electric mixer set over a pan of simmering water, combine egg whites and sugar. Whisk constantly until the sugar has dissolved and mixture is very warm to the touch, 3 to 5 minutes.

3. Attach the bowl to the electric mixer fitted with the whisk attachment; beat, starting on low speed and gradually increasing to high, until stiff, glossy peaks form, about 5 minutes. Add almond extract and continue beating until incorporated. Switch to the paddle attachment. With the mixer on medium-high speed, quickly add butter, a few tablespoons at a time, beating until mixture is smooth and has increased in volume. (If the frosting appears to separate after all the butter has been added, continue beating until smooth again.), Reduce speed to low, and continue beating for 2 minutes to eliminate any air bubbles.