Step 1

In the bowl of a stand mixer fitted with the whisk attachment, combine the sugar and egg whites and beat on medium speed to mix. The mixture will be very thick and grainy.

Step 2

Put 1 inch / 2.5cm of water in the bottom of a double boiler or a medium saucepan and bring to a gentle simmer over medium-low heat. Place the bowl with the sugar mixture over the simmering water and stir the mixture with a rubber spatula until the sugar is completely melted. Brush down the sides of the bowl. Feel the mixture between your fingers to check for graininess. Once all the sugar has melted and the mixture is smooth, the syrup is hot enough (140° F / 60° C) to be safe to consume.

Step 3

Return the bowl to the stand mixer fitted with a clean whisk attachment and beat on medium-high speed until the egg foam is light, fluffy, and glossy and the bowl feels just about room temperature. (If the egg foam isn't cooled sufficiently, the butter will melt when you add it.)

Step 4

Once the egg foam is whipped and cooled, turn the mixer speed to medium, add the butter, two tablespoons at a time, and beat until incorporated.* Turn the speed to low, add the vanilla and salt, and mix until incorporated.

Step 5

Use immediately.