I'm using Thomas Keller's marshmallow recipe. The syrup is cooked to 284 F / 140 C.
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Stephanie is the Head Recipe Tester of Food52.
I wouldn't recommend using simple syrup instead. The corn syrup acts like invert sugar to help prevent crystallization and effects the final texture of the marshmallows. You can make invert sugar syrup fairly easily and use that to substitute. Here's a good reference -- http://www.thekitchn.com...
Margie is a trusted home cook immersed in German foodways.
I am not sure why you want to substitute a simple syrup, but if you are trying to avoid high fructose corn syrup, the corn syrup you buy at the grocery--most likely Karo--is not HFCS. That is a more processed sweetener than corn syrup.