Beat the Egg Yolks

In a medium bowl, with a handheld mixer, beat the egg yolks on high speed until light in color.

Mix the Liquid Ingredients

Have ready a 1 cup glass measure with a spout, lightly coated with nonstick cooking spray.

In a small saucepan, preferably nonstick, using a silicone spatula, stir together the sugar and corn syrup until all of the sugar is moistened. Heat over medium-high, stirring constantly, until the sugar dissolves and the syrup begins to bubble around the edges. Stop stirring completely and continue cooking for a few minutes until the syrup comes to a rolling boil. (The entire surface will be covered with large bubbles. At this point, the temperature of the syrup on an instant-read thermometer, if using, should read 238°F/114°C.) Immediately transfer the syrup to the glass measure to stop the cooking.

Make the Buttercream

Beat the sugar syrup into the egg yolks in a steady stream. Do not let the syrup fall on the beaters or they will spin it onto the sides of the bowl. Continue beating on high speed for 5 minutes. Let it cool completely. To speed cooling, place the buttercream in an ice water bath or in the refrigerator and stir occasionally.

Complete the Buttercream

When the outside of the bowl feels cool, beat in the butter by the tablespoon on medium-high speed. The buttercream will not thicken until almost all of the butter has been added. Add the lemon zest, lemon juice, and vanilla and beat on low speed until incorporated. Raise the speed to high and beat until smooth and creamy. The buttercream may separate at first, but it will come together on beating.

Place the buttercream in an airtight container. Use it at once or set it aside for up to 4 hours. If keeping it longer than 4 hours, refrigerate it. Bring it to room temperature before using, to prevent curdling, and rebeat to restore the texture.