Lightly coat chicken pieces with flour. (Omit the wings and bony pieces.) Sauté in butter until golden, but do not brown. Pour fat from skillet, then add ¼cup Cognac and set aflame. When flame dies, add salt, pepper, stock, champagne, and sugar. Simmer about 40 minutes, or until done. Put chicken on hot platter. Thicken liquids with beurre manié.
Combine crème and yolk. Add about ½cup of hot sauce to this liaison, mix and then pour back into sauce. Heat but do not boil. Add 1tablespoon Cognac. Pour sauce through sieve over chicken.