Combine the buttermilk and salt in a non-reactive bowl. Be sure the fish is covered in the liquid. Soak it for 20-30 minutes.
Remove the salmon from the buttermilk brine, carefully rinse in cold water and gently pat it dry with some paper towels.
Mix together the salt, sugar, dried herbs, spices and freshly chopped fennel fronds. Set aside.
Place a piece of cheesecloth large enough to wrap the salmon in the bottom of a glass baking pan (Any non-reactive pan will do). Pour half of the curing mix onto the middle of the cheesecloth and spread it evenly. You should have about half an inch of cure or so.
Place the salmon on top of the cure mix, sprinkle the remaining cure mix on top of the fillet ensuring that it is covered completely.
Carefully wrap the salmon with the cheesecloth creating a sort of pouch; flesh side up.
Splash on the cognac over the flesh side of the salmon.
Place a piece of plastic cling film on top of the moistened cheesecloth and place a plate or shallow dish on top with a can inside for weight.
Place the salmon in the refrigerator, flipping it over every 12 hours, replacing the plastic cling film back on the top, replacing the weighted plate. Do this for at least two days.
Unwrap the now cured salmon from the cheesecloth. Carefully brush away the cure mix with a paper towel.
Slice thinly against the grain on a bias and serve. Suggestions are in your favorite warmed bagel smeared with cream cheese, sprinkled with capers, red onion and fresh dill.