This buttercream takes some time, but it is well worth it if you are a Buttercream lover. See what you think! Its amazing on cupcakes, sheet cakes, or even layered cakes.
Over low heat in a medium pot, bring one inch of water to a boil. Then turn down to simmer.
Make sure your stainless-steel mixer or bowl is clean and dry.
Place bowl over the pot of simmering water, not on top of the water, but have the bowl sit nicely on top of the pan to create a seal. In the bowl or pan over the simmering water whisk together the egg whites and sugar until the mixture reaches 160 degrees (about 3 minutes) or until sugar is completely dissolved. Mixture will feel hot to the touch.
Transfer bowl to the mixer stand or use a hand held electric mixer and continue to whisk until glossy peaks form. 15-20 minutes. The bottom of the bowl must be at room temperature before you add the butter.
Switch to the paddles attachment, or continue to beat with the electric mixer. Add the room temperature butter one tablespoon at a time. Butter cannot be too soft. If it is, put back in the refrigerator for a couple minutes. After the butter is all incorporated, beat the mixture until thick. Keep beating until any liquid is gone. Make sure it is thick and smooth.
Add the vanilla and fleur de sel and whip once again for a minute.
*You can make the Buttercream a day ahead, just make sure to refrigerate. Before using, let it come back to room temperature then rewhip until smooth in texture.