1 lb. unsalted butter
1 c. sugar
1/4 c. water
6 egg whites
1/8 t. salt
1 T. vanilla extract
1 scraped vanilla bean (opt.)
In a small sauce pan, combine sugar and water.
Try not to get any of the sugar/water mixture on the walls of the pan. Have a small bowl of ice water on the side.
The sugar mixture will bubble as you continue to heat it to 248 (firm ball stage on the candy thermometer). You can use a candy thermometer or you can carefully spoon a small amount of the hot sugar into the ice water. As the sugar mixture moves through the candy stages, it will hold a shape in the ice water. When the sugar starts to hold a shape in the ice water (softball) start to whip the egg whites.
A firm ball will feel soft and pliable between your fingers.
Add the hot sugar mixture in a steady stream with the mixer on high speed. Stream it in the space between the wisk and the side of the bowl. In this photo, the sugar mixture looks a bit dark, so do not be concerned if your sugar mixture is clear.
The mixture may look a little curdly at this point. Do not worry; the butter will soon blend in and you will have a fluffy, smooth buttercream. Add the vanilla.