This recipe come with thanks to J. Kenji Lopez-Alt's presentation of this on Serious Eats!
Combine egg yolk, water, lemon juice, and a pinch of salt in the bottom of a cup that barely fits the head of an immersion blender. Melt the better in a small saucepan over high heat, swirling constantly, until foaming subsides. Should be 220°F.
Place the head of the immersion blender into the bottom of the cup and turn on. With the blender constantly running, slowly pour hot butter into the cup. It should emulsify with the egg yolk and lemon juice. Continue pouring slowly until all butter is added (should take about 30 sec).
Sauce should be thick and creamy. Season to taste with salt, black pepper, and a pinch of cayenne pepper or hot sauce if desired. Serve immediately, or transfer to a small lidded container and keep in a warm place for up to 1 hour before serving. Hollandaise cannot be cooled and reheated. (Recipe♦412)