Makes about 3 cups. Difficulty
4 large egg whites 1 cup sugar Pinch of salt 3 sticks unsalted butter, at room temperature
In a large heat-proof bowl, combine the egg whites, sugar and salt. Place bowl over a pan of simmering water and stir constantly with a spatula until the sugar dissolves and reaches 140-150 degrees F. Remove the bowl from the pan of water.
Using an electric mixer, beat the egg white mixture at high speed until thick and cooled to room temperature (see tips above). Add the butter, a little bit at a time, and beat until the buttercream is smooth, thick and spreadable.
Make sure the meringue has cooled to room temperature before adding the butter or else you will create a soupy mixture. If it is too warm and it melts the butter, place the bowl of soupy buttercream over ice water and continue to beat with an electric mixer until smooth, thick and spreadable.