Place the egg whites in the bowl of an electric mixer fitted with the whip attachment.
Combine the sugar and water in a medium pot and stir over medium heat until it comes to a boil. When it begins to boil, stop stirring and attach a candy thermometer to the pot (if any sugar crystals have washed up on the sides, brush them away using a pastry brush dipped in cool water). Continue to cook the syrup until it reaches 240° F (final desired temperature).
As soon as the sugar hits 230° F, begin whipping the egg whites on medium-high speed. The goal is to have the egg whites at soft peaks when the sugar reaches the 240° F.
With the mixer running, add the sugar syrup in a slow, steady stream. Continue to whip on high speed until the mixture reaches stiff peaks and the bowl is no longer noticeably warm to the touch. The meringue should be smooth and glossy, not clumpy or dry.
With the mixer still running, gradually add room temperature butter in 1/2-tablespoon chunks. Continue adding and mixing until all of the butter is incorporated and the buttercream is light and smooth.
Beat in the vanilla and mix to combine. The buttercream can be used immediately or refrigerated in an airtight container.
I always have three kinds of hot sauce in my purse. I have a soft spot for making people their favorite dessert, especially if it's wrapped in a pastry crust. My newest cookbook, The Book on Pie, is out on November 10th, 2020.