Place a pot of water on the stove and bring to a simmer. Weigh your egg whites and sugar into your mixer bowl. Place your mixer bowl over the simmering water and whisk frequently until all of the sugar dissolves and the mixtures lightens in color. The mixture will be about 160 degrees F and you should feel no graininess when you dip your finger into the mixture and rub your fingers together.
Remove the mixer bowl from the heat and place on the mixer fitted with the whisk attachment. Beat the meringue on high speed until the mixer bowl feels cool to the touch. This can take 10 to 15 minutes. The mixture will be very white and glossy and will have tripled in volume.
Once the meringue is cool, turn the mixer to medium speed and begin adding the butter in chunks, allowing a few seconds for each addition to be incorporated. The meringue will look a bit deflated but just continue adding the butter and beating until all of the butter has been added. Continue beating the buttercream until it looks fluffy and creamy. Add the salt and vanilla and beat for 2 more minutes.
The buttercream can be covered and kept at room temperature overnight.
You can flavor the buttercream by adding melted chocolate, espresso or a fruit puree. Add it gradually, mixing well between additions, until you achieve the desired flavor being mindful not to add so much that you change the consistency.