Place egg yolk, water, lemon juice and pinch of kosher salt in a tall, narrow container that fits snuggly around the head of the immersion blender [this detail is important, so don't skip it!].

Place butter in a measuring cup (or other container with a pour spout), and warm in the microwave until hot (about 1 minute). I did 3 20-second bursts to keep the butter from popping and sputtering. Timing will depend on your microwave. If you don't have a microwave, you can easily warm the butter up on the stove in a small saucepan.

Place the immersion blender in the tall, narrow container with the egg yolk, and start blending. With the blender running, slowly stream in the hot butter. The butter will eventually emulsify with the egg yolk, forming a thick, creamy sauce. Season to taste with additional salt.

Hollandaise will keep for 1 hour at room temperature, and cannot be made in advance.