Melt the butter in the microwave for a few seconds.
Combine the egg yolks, mustard (if using), lemon juice and pepper in a blender and blend until combined.
With the blender running, slowly stream in melted butter and allow the sauce to emulsify. If the sauce is very thick, stream in a little warm water, a few drops at a time, until the sauce has reached the consistency of your choice.
Taste and adjust seasoning if necessary. I like using salted butter but if you are using unsalted, season with salt before serving.