In a medium saucepan, combine the egg whites, sugar, water, and cream of tartar.
Place the saucepan over medium heat, and stir continuously until the sugar is dissolved and the mixture is hot to the touch.
Remove the saucepan from the heat and transfer the mixture to a large bowl. Using an electric mixer, beat the mixture on high speed until stiff peaks form.
Reduce the speed to low and add the butter, a tablespoon at a time, beating until incorporated. Add the vanilla extract and beat until combined.
Use the buttercream immediately or refrigerate in an airtight container until ready to use.
Interesting Facts
Italian Buttercream is a type of meringue-based buttercream.
This type of buttercream is very stable and can be used for intricate cake decorations.
It can be made with different flavoring ingredients like extracts, zest, and liqueurs.