Hollandaise

From the Lexicon, Emulsions

Fat and water convinced to coexist as droplets, held by an emulsifier (egg-yolk lecithin, mustard, starch, or sheer agitation): the physics behind mayonnaise, hollandaise, vinaigrette, beurre blanc, aioli, and glossy pan sauces. The universal method: add fat SLOWLY to the water phase while shearing hard; the universal failure: too fast, too hot, or too cold, and it 'breaks' into grease. The universal rescue: start fresh with a new yolk (or a spoon of water/mustard) and whisk the broken sauce INTO it. Pasta's starchy water emulsifying oil and butter into clinging sauce is the same trick; Italian kitchens just never named it.