Reheating hollandaise

Try using a double boiler (low, indirect heat)

I freeze extra hollandaise and then reheat it over low low heat on the stove. Depending on the amount of leftovers, it helps to whisk some melted butter and water into the pan

Just heat slowly at a low temp.

You have to be very careful when reheating hollandaise, you could scramble the eggs if you heat on high heat. I have reheated it before by placing in a bowl on top of simmering water stir constantly until its warm. Maybe some of our resident experts have a a better method but this worked for me.

Michael Ruhlman says you can warm it in the microwave and then re-emulsify it with a little water. I'd just microwave it a few seconds at a time though.