Swiss-style Buttercream Icing with Variations

I have been on a quest to find  what I am willing to call my go-to buttercream frosting.  I love this chocolate frosting, but a true go-to for the rest I haven't found yet. Possibly until now!  I have made this several times now and it's been a hit.  I also love that there are lots of variations readily available.  (This recipe is from Cupcakes by Shelly Kaldunski.)

- 3 large egg whites, at room temperature

- 1/4 cup sugar

- pinch salt

- 1 cup unsalted butter, cut, at room temperature

- flavorings

Combine the egg whites and sugar.  Set the bowl over (but not touching) simmering water in a saucepan and heat the mixture, whisking constantly, until the sugar has completely dissolved and the mixture is very warm to the touch, about 2 minutes.  Remove the bowl from the saucepan.  Using an electric mixer on high speed, beat the egg white mixture until it is fluffy, cooled to room temperature, and holds stiff peaks (the mixture should not look dry), about 6 minutes.

With the mixer on medium-low speed, add the salt and the butter, a few pieces at a time, beating well after each addition.  If the frosting appears to separate or is very liquid after all the butter is added, continue to beat on high speed until it is smooth and creamy, 3-5 minutes more.  Add the flavoring(s) of choice and beat until combined, scraping down the sides of the bowl as needed.  Use right away, or store in an airtight container for up to 5 days.  (If refrigerated, bring it to room temperature and beat it at medium-low speed until smooth.)

Flavorings:
* Chocolate - 2 tbsp unsweeetened cocoa powder whisked into 4oz semisweet chocolate, melted and slightly cooled

* Coffee - 1/2 cup strong brewed coffee, at room temperature

* Rum - 1 1/2 tbsp dark rum (I really want to try this one!)

* Raspberry or Strawberry - 1/3 cup strained preserves and 6 drops red food coloring

*Vanilla - seeds from 1/2 vanilla bean or 1 tsp vanilla extract