Add the egg whites and sugar and whisk 7 mins/60/speed 2.5, MC on.
Meanwhile, weigh your butter into a separate bowl and set aside. (Unless your kitchen is very hot. If it is, place butter in a cooler place)
Remove MC and beat again 3mins/speed 3.5.
Check that the egg white has whipped to the point where it will form firm peaks. if your eggs were straight from the fridge, it may need another couple of minutes.
Allow to cool until the temperature is below 37C.
Set machine to 5 mins/speed 3 and add vanilla, then begin adding the cubes of butter, one at a time. This should take about 4 minutes.
Place bowl in the fridge for half an hour to cool down.
To bring back to piping or spreading consistency, mix for 4 secs/speed 3.
Use as desired.
Notes
For Chocolate Swiss Meringue Buttercream, add 20g cocoa part way through adding butter at stage 7. Scrape down to incorporate all the cocoa and continue mixing.
Instead of cocoa, try adding 50g melted and cooled chocolate. For a white chocolate buttercream reduce the sugar by 50g and add 50g melted and cooled white chocolate.
Another couple of flavours could be achieved with zested orange or lemon rind and a few drops of either orange or lemon essence.
I used a dash of rosewater, some pink food colouring decorated with some Pashmak (Persian Fairy Floss) and tiny dried rose buds to make a very special cake for my daughter in law. This will keep in the fridge for up to three weeks. It can also be frozen - defrost in the fridge for 24 hours. Before using, whip it up with the butterfly on at speed 3.5 until it reaches the consistency you're after.
Recipe by ThermoGourmand at https://thermogourmand.com.au/index.php/2020/10/26/chocolate-swiss-meringue-buttercream/