It's primal and it's easy ... don't be fooled that this sauce is complex, as it is not.
living in the ice age: Pink Grapefruit Mayonnaise
Put the egg yolks in a bowl which should be large enough to sit over a pan of water without touching the water. This is called a baines marie.
Squeeze some lemon juice in. A tablespoon is good, or just a squeeze of a segment. I often use grapefruit, particularly pink grapefruit, which works very well indeed over squid!
Whisk together and whisk together well. Taste it. It should be sour.
Melt some butter in another pan and allow the solids to fall to the bottom.
Whisking the eggs continuously as your pour the melted butter in, taking care not to get too much solid in. Don't worry - you're not going to turn the eggs to custard, as the cold mixture and cold pan will cool things down.
Bring a pan of water to the boil and place the bowl over - take a lot of care at this point and don't take your eye of it for a second. Whisk as the mixture heats through.
If it gets stiff, or looks like it's turning to scrambled egg, just take the bowl off the heat and add a splash of cold water - literally, a splash.
Once don't just pour over whatever it is you want to ...
Here's a tip: living in the ice age: Eggs Benedict with Shaved Truffle ... or living in the ice age: Eggs Hemingway or even living in the ice age: Eggs Florentine
If you find yourself making this up because you love it (I do) and have egg whites left over, you could make up a MEGA-EGG! living in the ice age: Leftover Egg White Breakfast
Have fun ...