Poach an egg for 1-2 minutes, just enough that the white is cooking but the yolk is still soft.
Melt butter until boiling hot.
Add the poached egg to a blender.
Add about a teaspoon of mustard.
Add roughly a teaspoon of white vinegar (less for less tangy, more for sharper).
Add a couple tablespoons of hot water (more for thinner mayo, less for thicker).
Add a good-sized pinch of salt.
Blend on low speed.
Slowly pour the boiling butter into the blender while it is running.
Crank the blender to high speed.
Pour the mixture into the refrigerator and let it set up overnight.
