In a stand mixer fitted with a whisk attachment, beat the egg yolks on medium-high speed until they become thick and pale in color.

In a small saucepan, combine sugar and water over medium heat. Stir until the sugar dissolves, then stop stirring and let the mixture cook until it reaches 238°F (114°C) on a candy thermometer (soft-ball stage).

With the mixer running on low speed, carefully pour the hot sugar syrup into the beaten yolks in a thin, steady stream.

Increase the speed to high and continue beating until the mixture cools to room temperature, about 5-7 minutes.

Reduce the mixer speed to medium and gradually add the softened butter, one tablespoon at a time, allowing each piece to fully incorporate before adding the next.

Once all the butter is incorporated, add vanilla extract and a pinch of salt. Continue mixing until smooth and fluffy.

Use immediately or store in the refrigerator for up to a week. Re-whip before using.