Place egg whites and sugar in a metal bowl of a stand mixer. Place bowl over a pan of simmering water, not touching the water. Whisk or stir constantly until sugar is dissolved and mixture comes to approximately 160ºF.
Immediately place the bowl on the stand mixer and whip on high using the whisk attachment until frosting has become thick and glossy, forming a stiff peak. Continue to whip frosting until the bowl feels room temperature to the touch, a total of about 10 minutes.
Switch the whisk attachment to the paddle attachment. With the mixer on low, drop in butter, 1 tablespoon at a time, beating until smooth after each addition. If frosting appears curdled or clumpy at any time, increase speed to medium-high and beat until smooth.
Once all of the butter has been incorporated, mix in extracts and pinch of salt. If needed, continue beating until smooth.
Swoop and dollop the Swiss Meringue Buttercream on top of the cooled cake and add the sliced citrus on top