In a deep pot fill ¾ full of water and bring to the boil.
In a small pan Bring to boil the tarragon vinegar and onion reduce by 1/2.
Leave to the side to infuse.
Melt the butter and allow to stand and separate the butter oil from the butter milk Ideal temperature 38℃/98℉.
Place the egg yolks in bowl with the lemon juice salt and cayenne pepper.
Add a dash of boiling water from the pot.
Turn off the heat of the pan with boiling water.
Place the egg mix in the bowl on top of the hot water pot.
whisk the egg mix until light fluffy and thick.
Continue whisking the egg mix for 1 minute, and slowly add the butter oil in a steady stream.
Strain the vinegar mix through a sieve and whisk together the vinegar and egg and butter mix.
Whisk until all the butter oil has been added the sauce should be thick and smooth leaving ribbon like trail on the surface.
Check seasoning and use as required