Place egg yoke, salt, paprika, sugar, cayenne, mustard, vinegar and lemon juice in a blender.
Cover and blend for 15 seconds.

Remove lid (if you have a little screw-out bit just remove that and put a funnel in through the hole).
Add the oil - pour steadily as the blender is blending. Don't take too long or the whole thing heats up.

Add the boiling water and blend for a couple of seconds.

Store in a screwtop jar in the refrigerator.