How hollandaise works
The key to mastering hollandaise sauce is understanding emulsification. As you whisk the sauce, you’re separating the butterfat into very, very small droplets (called the “dispersed phase”), and spreading them out through a certain amount of water (called the “continuous phase”) so they can’t recombine. It’s the same thing you do when making mayonnaise, or a proper vinaigrette.
What makes hollandaise especially tricky is that butterfat is a solid (though a soft one) at room temperature, so you have to make sure it’s warm in order for it to be fluid enough to emulsify.
Because there’s heat involved, there are two big things that can go wrong: heat the sauce too much, and the egg proteins will coagulate, leaving you with very buttery scrambled eggs. If this happens, there’s not much you can do but start over with fresh eggs. (Though you can strain the cooked eggs out of the butter, and reuse the butter.)
Similarly, heat will cause the water in the sauce – whether in the form of vinegar, lemon juice or just plain ol’ water – to evaporate. If there’s not enough water, the droplets of butterfat will pool together, and you will end up with what’s known as a “broken” sauce. (The same thing happens if you add too much butter; it’s the ratio of butter to water that’s the important thing.)
To prevent this from happening, make sure you keep the heat steady but low (keep the sauce below 160°F to prevent the eggs from curdling). A double boiler is the easiest way to do this. And if the sauce gets shiny, and it looks like oil is starting to seep out, add a small splash of water to maintain the emulsion. If you don’t, your sauce will break.
The good news is that it’s easy to fix a broken sauce. More on this later.