Prepare the Egg Yolks: In a heatproof bowl, whisk the egg yolks until smooth.

Make the Sugar Syrup: In a small saucepan, combine the sugar and water. Heat over medium-high heat, stirring occasionally, until the mixture reaches 240°F (115°C) on a candy thermometer.

Temper the Eggs: Slowly pour the hot sugar syrup into the bowl with the egg yolks, whisking constantly until well combined.

Whip Until Cool: Using a stand mixer or electric hand mixer, whip the egg yolk mixture on high speed until it has cooled to room temperature and doubled in volume.

Incorporate Butter: Gradually add the softened butter, one cube at a time, mixing well after each addition until smooth and creamy.

Add Vanilla: Mix in the vanilla extract until fully incorporated.

Use or Store: Use the Irish Buttercream immediately to frost your cakes, or store it in an airtight container in the refrigerator for up to one week.