1teaspoonDiamond Crystal kosher saltor ½ teaspoon sea salt
8tablespoonsunsalted butteror clarified butter
Add the egg yolk, water, lemon juice, kosher salt, and cayenne pepper into a cup that just fits the head of an immersion blender. I use a 16-oz coffee mug. You can also use the cup that comes with the blender if it's narrow enough.
Cube the butter, and place it in a glass measuring cup. Cover and melt the butter in the microwave, in 30-second increments. The butter should be hot, not just melted - see notes below.
Place your immersion blender into the bottom of the cup and turn it on. With the blender running, very slowly pour the hot butter into the cup. The butter will emulsify with the egg yolk and lemon juice, resulting in a thick, creamy sauce.
Transfer the sauce to a serving dish. Serve immediately. This sauce cannot be cooled and reheated, but it will keep for up to one hour at room temperature.
Slightly adapted from Serious Eats.The CDC recommends cooking eggs until both the yolk and white are firm. You want the butter to be hot, not just melted. If you worry about it splattering in the microwave, you can heat it up in a small saucepan on the stovetop over medium-high heat until it's melted and the foam subsides.To further increase your chances of success and reduce the risk of the sauce separating, try using ghee (clarified butter) instead of butter. Since clarified butter contains no water, it's easier to stabilize and thicken the sauce when using it.For the same reason, you can skip the water. Add it later if needed, if the sauce is too thick.