Beat the egg whites, salt, and cream of tartar, slowly add in 1/3 cup of sugar and continue beating until soft peaks form.
While the eggs are getting whipped add the remaining sugar and 1/3 cup water into a medium pot and place on medium-low heat.
Stir until sugar melts and becomes clear.
Maintain at medium-high heat until temperature reads 235-240F.
Drizzle the sugar into the mixer immediately. At this point, the meringue should be at the soft peak stage.
Run mixer until meringue is cool/tepid.
Switch to a paddle attachment. Add room temperature butter into running mixer one tablespoon piece at a time.
Add the salt and vanilla if using.
Beat until butter is combined and mixture has reached a silky consistency.