This egg white buttercream is the easiest of the classic French buttercreams because it does not require a cooked syrup. A hand-held electric mixer is necessary; be especially careful to keep the cord away from the burner. Use a stainless-steel bowl, rather than glass or crockery, to ensure that the meringue is adequately heated. Be sure to rinse the stem of the thermometer in the simmering skillet water between readings, to avoid contaminating the egg whites.
Stir together in a small bowl until dissolved and set aside:
1 tablespoon instant coffee crystals or instant espresso powder
1 1/2 teaspoons water
Have the butter and egg whites at room temperature, 68┬░ to 70┬░F. Whisk together in a large stainless-steel bowl:
4 large egg whites
3/4 cup sugar
2 tablespoons water
1/4 teaspoon cream of tartar
Set the bowl in a wide, 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 on low speed until the mixture reaches 140┬░F on an instant-read thermometer. Do not stop beating while the bowl is in the skillet, or the egg whites will be overcooked. If you cannot hold the thermometer stem in the egg whites while continuing to beat, remove the bowl from the skillet just to read the thermometer, then return the bowl to the skillet. Beat on high speed just until the mixture reaches 160┬░F, 2 to 4 minutes. Remove the bowl from the skillet and add:
1 teaspoon vanilla
Beat on high speed for 3 to 5 more minutes, to cool. The mixture should hold glossy marshmallowy peaks. In another large bowl, beat until creamy, about 30 seconds:
3/4 pound unsalted butter
Beat a large dollop of the meringue into the butter until well combined. Continue to beat in about half of the meringue in large dollops. Scrape the remaining meringue into the mixture and beat until smooth and fluffy. Beat in most of the coffee mixture, then add more to taste along with:
1 to 2 tablespoons liqueur (optional)
This keeps, refrigerated, for up to 6 days. Or freeze for up to 6 months. Soften as for Classic Buttercream.
