Place your egg yolk, cayenne pepper, salt and lemon juice in the blender and blend about 30 seconds.
Slowly add your melted butter, just a little at a time, blending until all butter has been incorporated and the sauce has thickened (about 30 seconds to 1 minute usually).
Serve immediately.
Notes
Pour the butter in very slowly while pureeing to allow the sauce to emulsify.
Adjust the amount of butter depending on how thick you want your sauce (add more butter for thinner sauce, less for thicker).
Clarified butter will work great in this recipe, but it's not a requirement.
If the sauce has broken (butter and egg yolks separate), blend it for another 20-30 seconds. You can also add 1-2 tablespoons of hot water to help thicken it.
This recipe uses raw egg yolks which can cause foodborne illnesses. You can pasteurize eggs using the sous vide method by cooking them in a water bath at 135°F for 75 minutes. The place them in an ice bath for 30 minutes to cool and use like normal.
You can also make this in a regular blender as long as the blades are down low enough.