Combine the egg yolks in a heatproof mixing bowl and set aside. In a saucepan, combine sugar and water, then cook over medium heat, stirring until the sugar dissolves. Attach a candy thermometer and heat until the syrup reaches 240°F (soft ball stage).
4 large egg yolks, 1 cup granulated sugar, 1/2 cup water
Once the syrup reaches 240°F, carefully pour it in a thin, steady stream into the bowl with the egg yolks while whisking constantly. Continue whisking until the mixture thickens and becomes pale, glossy, and tripled in volume, approximately 5-7 minutes.
4 large egg yolks, 1 cup granulated sugar, 1/2 cup water
Remove the bowl from heat and continue whisking until the mixture cools to room temperature, observing a light, fluffy, and airy texture.
Gradually add softened butter, one piece at a time, while continuously whisking. Wait until each addition is fully incorporated and the mixture smooths out before adding the next.
1 cup unsalted butter
Once all the butter has been added, continue whisking until the buttercream is smooth, shiny, and creamy—about 2-3 minutes. The buttercream should hold stiff peaks when lifted.
1 cup unsalted butter
