Start by preparing the egg yolks: In the bowl of a stand mixer, add the 5 large egg yolks. Using the whisk attachment, beat the yolks on high speed until they become thick, pale, and slightly increased in volume. This should take about 5 minutes. The yolks will transition from a bright yellow to a pale yellow, and they should resemble a thick, smooth ribbon when the whisk is lifted from the mixture. This step is crucial for the structure of the buttercream, so be sure to let it fully develop to the right consistency.
Make the sugar syrup: While the egg yolks are beating, combine 1 cup (200g) of granulated sugar and 1/4 cup (60ml) of water in a medium-sized saucepan. Stir occasionally to dissolve the sugar, and once dissolved, stop stirring. Let the mixture come to a boil over medium heat. You should monitor the syrup carefully as it boils. Using a candy thermometer, allow the syrup to cook until it reaches 238°F (114°C), which is the soft-ball stage. At this stage, a small drop of the syrup should form a soft ball when dropped in cold water. Be careful not to stir the syrup once it begins boiling, as this can cause crystallization.
Combine the syrup with the egg yolks: Once the syrup reaches the correct temperature, turn the mixer speed down to medium. Slowly and carefully pour the hot syrup into the whipped egg yolks in a thin, steady stream. Avoid letting the syrup hit the whisk, as it can cause splattering. The syrup will cook the egg yolks as it’s added, and they will thicken further. This step ensures that the egg yolks are incorporated properly with the syrup, giving the buttercream its rich texture.
Whip the mixture to cool: After the syrup is added, increase the mixer speed to high. Continue to whip the mixture for about 5-10 minutes, or until it becomes thick, glossy, and cool to the touch. You’ll notice the mixture turning from a liquid to a thicker, more stable consistency as it cools. To check the temperature, you can touch the outside of the mixing bowl—when it feels cool, the mixture is ready for the next step. This cooling process is important to prevent the butter from melting when it’s added to the mixture.
Add the butter: Once the mixture has cooled to room temperature, reduce the mixer speed to medium. Begin adding the 1½ cups (340g) of room-temperature unsalted butter, one piece at a time. Allow each piece to fully incorporate before adding the next. The butter will create a smooth, silky texture, and the mixture will become even thicker. It’s important to add the butter slowly, as this allows the frosting to emulsify properly. If the mixture starts to look soupy or curdled, don’t worry! Simply continue mixing, and it will come together into a smooth, creamy buttercream.
