For the cod back fillets, season them with salt and cook in an unperforated cooking container (at 100°C for 4-6 minutes).
Meanwhile, finely chop tarragon and dice shallot. Sauté half of the tarragon leaves and the shallot cubes with butter on the stove top.
Add vermouth and reduce by half. Strain the sauce and return to the pot.
Add sour cream and bring to a boil. Stir in tarragon vinegar and season with salt and cayenne pepper.
Before serving, add the remaining tarragon leaves. Pour the sauce over the fish and serve.