Put a saucepan with 1 to 2 inches of water on medium high and bring to a simmer
In a grease-free mixing bowl, add the egg whites, salt, and sugar and whisk lightly to combine.
Set the bowl over the saucepan with the simmering water and heat whisking continuously until it reaches 160 degrees or until it no longer feels grainy between your fingers.
Whisk mixture on medium-high speed in the mixer until stiff peaks and mixture is to room temperature. It should be thick and glossy.
Remove whisk attachment and replace with the paddle attachment.
Set the mixer to low and slowly add the butter one tablespoon at a time.
Keep beating until the buttercream thickens and reaches a smooth consistency. It may look curdled at some point but keep beating and it will come together.
Add vanilla, and bourbon/rum. Mi¼x to combine.