Makes about 275g of buttercream, or 1½ cups. You will need a candy or sugar thermometer (or a multimeter) for this recipe.
Ingredients
120g (or ½ cup + 5 teaspoons) granulated sugar
3 tablespoons water
2 large egg whites (about 65g or ¼ cup)
113g (or ½ cup) unsalted butter, cubed and softened at room temperature
½ teaspoon vanilla extract
Instructions
Combine sugar and water in a medium-sized saucepan. Heat over low heat, stirring with a metal spoon until the sugar has dissolved and the syrup is clear. Crank up the heat to medium-high and allow syrup to come to a boil.
In the meantime, place egg whites in a medium-sized, heatproof bowl and mix (with a mixer fitted with the whisk attachment) until the whites are almost able to hold soft peaks.
Once the sugar syrup has come to a boil, clip on a candy (or sugar) thermometer or insert the probe of a multimeter. Cook until the syrup reaches a temperature of 113°C/235°F, then immediately remove from the heat and slowly drizzle the hot syrup into the bowl with the egg whites, mixing continuously to prevent the eggs from scrambling. Don't pour the syrup onto the whisk, or the syrup may splatter against the sides of the bowl (or into your face!). Instead, aim for a spot close to the whisk.
Once all the syrup has been added, keep mixing until the bottom of the bowl feels cool to the touch and the meringue has cooled down to at least body temperature.
Start adding the butter to the meringue one cube at a time, mixing well after each addition. As you're adding the butter, the mixture will lose most of its volume and at some point may even start to look as if it's separating. Don't panic! Just keep mixing and the buttercream will magically come together and become smooth and creamy. Once the buttercream has come together, mix in the vanilla.
Use immediately or store in an airtight container or a zipper bag in the fridge for up to two weeks or in the freezer for up to two months. To use buttercream that has either been refrigerated or frozen, first allow to come to room temperature then beat with the mixer until smooth and spreadable again. Cakes or cupcakes decorated with buttercream generally keep 3-5 days, stored in an airtight container in the fridge. Allow cakes and cupcakes decorated with buttercream to come to room temperature before serving, because this improves the texture of the buttercream. For a big cake, this may take up to 4 hours.