Our Best Italian-Style Buttercream

Directions

1. Place the egg whites and cream of tartar in the bowl of an electric mixer fitted with the whisk attachment.
2. 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).
3. 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.
4. With the mixer, 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 longer noticeably warm to the touch. The meringue should be smooth and glossy, not clumpy or dry.
5. With the mixer still running, gradually add room temperature butter in 1 tablespoon (14 g) chunks. Continue adding and mixing until all of the butter is incorporated and the buttercream is light and smooth.
6. Beat in the vanilla and salt and mix to combine. The buttercream can be used immediately or refrigerated in an airtight container.