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Stephanie is the Head Recipe Tester of Food52.
What are you trying to make? Eggs will coagulate at less than 250 degrees. For things like a buttercream, you have to slowly temper the hot sugar syrup into the whipping eggs so that they don't coagulate. The easiest way to do it is a stand mixer and trickle the syrup slowly down the side of the bowl into the whipping eggs.
I guess you are asking about European buttercreams. Finecooking magazine has had some fine articles on making Swiss and Italian style. They make have it available free on finecooking.com
Cynthia is a trusted source on Bread/Baking.
Whether you're making an Italian buttercream with egg whites or a French one with yolks, as Stephanie says, it is critical that the sugar syrup be poured very slowly down the side of the mixing bowl with the whisk rotating on high speed. The syrup must not hit the whisk, or it will spin a mess you'll wish you could fling into outer space. Persevere! The first time you get it right, you'll feel like you should win a prize.
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