Step 1In a heavy-bottom medium saucepan, combine the sugar and corn syrup with one-half cup water and mix together to wet the sugar. Bring the mixture to a boil and cook until a candy thermometer inserted reads 238 degrees. Immediately remove from heat to stop the cooking process.
Step 2While the sugar is cooking, beat the eggs to soft peaks in a stand mixer using the whisk attachment, or in a large bowl with a hand mixer.
Step 3With the mixer running, beat the sugar syrup into the egg whites to form a meringue and continue beating until the meringue cools to room temperature, 10 to 15 minutes. (It is important that both the meringue and butter be at room temperature or the buttercream will separate.)
Step 4With the mixer running, beat in the butter, 1 to 2 pieces at a time, until all of the butter is incorporated. The buttercream can be made and refrigerated, covered, a few days in advance but should come to room temperature before it is used (beat the buttercream once more just before using to make sure it is emulsified).