Zabaione (also called zabaglione) and hollandaise sauce are both made on a double boiler (ideally a metal bowl placed over a pot of water — glass is a poor conductor of heat).

Zabaione is much easier to master, and it is simply a mixture of the following ratio: one egg yolk, one tablespoon table sugar, and one tablespoon of a spirit (traditionally marsala). Simply combine them in the bowl placed over heated water (a little under boiling) and whisk them to produce a thick foam. Once they reach around 180°F (82°C) they should be taken off the heat, put into fancy bowls, and served immediately (while still warm). Note: some people cook zabaione to a lower temperature for a thinner product. I would cook to at least 160°F to ensure food safety. 

Hollandaise is one of the traditional Mother Sauces, and it is much more difficult to master using the traditional technique. You start with an egg yolk and a little acid (typically lemon juice) in the same setup as with zabaione (a metal bowl over a pot of water a little under boiling) and while whisking, you slowly and steadily stream in melted clarified butter (although melted whole butter can be used instead ,and it's actually a little easier). It's important not to add the butter too quickly or the emulsion will break. If that happens, you can start with a new egg yolk in a clean bowl and then just add in the broken sauce gradually. There are other ways to fix a broken sauce as well, but that is the easiest method.

In both cases, it's easiest to use an induction burner rather than gas. Gas burners cause the sides of the bowl to heat up too much, and the eggs can over-coagulate.

If you are using a gas burner, just bring the water to a boil then turn off the gas and then place the bowl on top to start making your sauce. If you use enough water (at least 1-2") there should be plenty of heat in the water to fully cook either sauce (note the water should not touch the bottom of the bowl). If you're making a large batch and/or it's taking a long time you can remove the bowl, bring the water back to a boil, and then turn off the gas again and return the bowl to the top of the pot. It's a good idea to continue whisking occasionally during this time to maintain your foam and emulsion.

Note that hollandaise sauce can't be chilled and reheated (unless you use additional stabilizers), so make it right before service to your guests and enjoy all of it.

Please let me know if you try this and how it works! I would definitely start with zabaione, since it's delicious and very easy to make successfully.

Chef these are SUPER helpful, thank you!