Add evaporated cane syrup and fresh egg whites to the bowl of a stand mixer.
Place bowl over double boiler, or atop a single pot of boiling water so long as the bowl is 1-2 inches above water.
Whisk sugar and egg whites until it reaches a temperature of 155-160 F.
Move bowl to the stand mixer and using the whisk attachment, mix on high speed until the bowl cools completely and the meringue forms stiff peaks. It should be just slightly thicker than the consistency of shaving cream. Depending on the mixer this can take anywhere from 5-10 minutes.
Reduce speed to medium-low.
Add room temperature butter 1-2 tbsp at a time ensure that it is fully incorporated before adding more. *Meringue will deflate at this time. Just before all butter is adding the buttercream will appear to be curdling. Don’t Panic! It will come together.
After all butter is incorporated, increase speed to med-high and mix for another minute.
Add vanilla and tiny pinch of salt and mix just until incorporated.
*Optional but recommended for cakes – Remove bowl from stand and use a rubber spatula to gently mix and fold the buttercream to get the air bubbles out.