Langoustines w/ Mayonnaise

In a large Dutch oven prepare a cooking broth:
2 l water, 10 cl of vinegar, coarse salt, and cayenne pepper.
Bring to a boil with a lid and during this time rinse 1.5 kg of langoustines in cold water.

When the water boils, throw in 1/3 of the langoustines.
Let them cook without lid and hard boil until they rise to the surface.
Skimmer them out as they come up and set them aside.
Repeat for the other two thirds.

Let cool and during this time prepare the mayonnaise:
Whenever possible, first bring all ingredients to room temperature.
In a high-edge bowl, put the extra-fresh egg yolk (less than 9 days), 1 tbsp. mustard and 1 hint of salt.
Mix well (mixer at medium speed).
Then add, while continuing to mix, 20 cl of oil progressively, (preferably choose mainly a vegetable oil with neutral taste which can be supplemented by a stronger oil).
If you pepper, prefer a white pepper (avoid small black dots).
Keep beating to thicken the mayo.
At the last moment, add a little oil and just mix (gives shine).
Do not store.
(The refrigerator, necessary after a while, disintegrates the mayonnaise because the oil freezes).

Presentation :
Put the langoustines side by side.
You can put some of them in evidence by turning the claws backwards so that they pinch their tail ("swallow" presentation).
Offer the mayonnaise on the side.

Langoustines (Nephrops norvegicus) are sometimes named "Norway lobsters" or "Dublin bay prawns". In France they are sometimes nicknamed "demoiselles". Buy langoustines about of the same size (to get an even cooking), and alive if possible.
Here langoustines don't have to be huge (unlike recipes where they are breaded). Prepare finger-bowls (water w/ lemon) and offer nutcrackers for the claws.
Freshness of the langoustines and cooking precision will make savor and texture.