Source: Martha Stewart Weddings
- 1 cup frozen black currant puree
- 5 large egg whites
- 1 1/4 cup sugar
- 1 pound unsalted butter, room temperature, cut into 8 pieces
- 2 teaspoons vanilla extract
- 1 tablespoon Creme de Cassis (black currant liqueur), optional
Spoon frozen black currant puree into a small saucepan. Cook gently on medium-low heat until reduced by half, 20 to 40 minutes; let cool. Transfer to a glass container with lid and refrigerate up to 1 day before use.
Place egg whites and sugar in the heat-proof bowl of an electric mixer. Set bowl over a pan of simmering water. Whisk the egg whites and sugar until sugar is dissolved and mixture registers 140 degrees on an instant-read thermometer, 2 to 3 minutes.
Transfer bowl to a mixer fitted the whisk attachment and beat hot egg whites on high until fluffy and cooled, 8 to 10 minutes, and until the underside of the bowl is at room temperature.
Reduce speed to medium-low and add butter, 1 piece at a time, mixing well after each addition. (Note: If buttercream develops a curdled appearance, it means the meringue mixture and butter were not at the same temperature before combining. To correct, place mixing bowl over a hot-water bath and stir with a rubber spatula. Mixture will come together.)
Add vanilla extract and Creme de Cassis, if desired. Add 2 tablespoons of the cooled black currant puree; adjust to taste.