Cook the crayfish in court-bouillon or salted water for 4 minutes, then cool. Separate heads from tails of 28 crayfish, reserving 4 whole. Reserve heads, peel tails, and keep warm.
Heat olive oil in a saucepan. Brown crayfish heads well, add carrot, onion, and celery. Deglaze with cognac, reduce, add white wine and bouquet garni. Boil 5 minutes, add 2 liters water, simmer 20 minutes. Strain through fine sieve, pressing solids. Reduce stock to 20 cl (2 glasses). Set aside.
In sauté pan, heat champagne and sugar until caramelizing. Add cream and crayfish stock. Reduce further.
Arrange peeled tails in porcelain egg cups on plates, top with whole crayfish. Warm in hot oven 2 minutes.
Reduce sauce by half, season with pepper and salt. Pour over crayfish, garnish with chervil sprigs.

Note: Photo shows crayfish in eggshells for pro presentation, but it takes extra work!
