This frosting stars in Wendy Kromer's Lemon Wedding Cake. If you have any leftover black currant reduction, add some to the simple syrup, use in cocktails, or drizzle over ice cream.
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.