Add egg whites and sugar to a medium-sized heatproof bowl (or the bowl of a double boiler) and whisk until combined. Set the bowl over a pan of simmering water, making sure the water doesn’t touch the bottom of the bowl, and gently heat the egg mixture, whisking continuously, until the sugar has dissolved. You can check whether the sugar has dissolved by rubbing a bit of the mixture between your fingers; you should no longer feel any sugar crystals. At this point, you can either pasteurize the mixture, or use the mixture unpasteurized.
If you want to pasteurize the mixture, continue to heat it, whisking continuously, until the mixture reaches a temperature of 71°C/160°F. You will need a sugar or candy thermometer, or a multimeter to properly register the temperature. If you decide not to pasteurize the egg whites, proceed with the next step.
Remove the egg white mixture from the heat and, using an electric whisk fitted with the whisk attachment, beat the mixture until the resulting meringue holds stiff peaks and has cooled to room temperature. The bottom of the bowl should also have cooled to room temperature.
Beat in the butter:
Add the cubed, softened butter one cube at a time, mixing well after each addition. If the mixture looks like it it’s separating after you’ve added all the butter, don’t panic. Just keep beating the mixture and the buttercream will magically come together again. If the buttercream is too soft to pipe, the butter was too hot when you added it. In that case, place the bowl with the soft buttercream in the fridge for about 20 minutes, then mix again. The buttercream should firm up.
When the buttercream looks smooth and creamy, beat in the strained cassis or blackcurrant jam one tablespoon at a time and mix until well combined.
Use the buttercream immediately (or within a few hours) or store in an airtight container in the fridge for up to 2 weeks. Allow buttercream that has been refrigerated to come to room temperature, then beat with the mixer until smooth and spreadable again before using it.
Recipe by The Tough Cookie at https://thetoughcookie.com/2016/02/14/fruity-cassis-buttercream-swiss-style/