Step 1: Prepare the Sugar Syrup

1. In a small saucepan, combine the sugar and water.
2. Heat over medium-high heat, stirring until the sugar dissolves.
3. Once dissolved, stop stirring and bring to a boil until the mixture reaches 240°F (115°C) (soft-ball stage) on a candy thermometer.

Step 2: Whip the Egg Yolks

4. While the syrup is heating, place the egg yolks in a stand mixer bowl.
5. Begin whisking on medium speed until the yolks become pale and slightly thickened.

Step 3: Incorporate the Syrup

6. With the mixer running on low speed, slowly pour the hot syrup into the yolks in a thin stream.
7. Once fully incorporated, increase the speed to high and whip until the mixture is thick, glossy, and cooled to room temperature (about 5 minutes).

Step 4: Add the Butter

8. Reduce the mixer speed to medium and add the butter, one cube at a time, allowing each piece to fully incorporate before adding the next.
9. Continue mixing until the buttercream is smooth, fluffy, and fully emulsified.

Step 5: Use & Store

10. Use immediately or store in an airtight container in the fridge for up to 1 week. Bring to room temperature and re-whip before using.