Melt the butter and let it cool slightly. Place eggs yolks, salt, pepper, and 1 tbsp lemon juice in the blender jar.Cover the jar of the blender and blend the egg yolk mixture at top speed for 2 seconds. Uncover, still blending at top speed, and immediately start pouring the hot melted butter in a thin stream.
You can add some hot water, a Tbsp at a time, if it is too thick.
Use immediately. You can set the blender jar in lukewarm water if you are not going to use it immediately.