Sprinkle each piece of cod with 7 g of salt on all sides including the skin. Wrap in cling film and marinate in the refrigerator for 1.5 hours. Rinse in cold water and pat dry.
Preheat oven to 200°C. Bake the potatoes (skin on) until soft, about 1½ hours. Remove the skin and push the flesh through a fine sieve while still hot (there should be about 250 g of pulp). Cool to room temperature. Add the egg yolk, flour, fresh herbs and some salt and knead until a dough is formed. Shape the gnocchi into evenly sized ovals and mark with a fork. Cook in boiling salted water until the gnocchi rise to the surface, about 3 to 5 minutes. Refresh by placing into iced water and set aside until needed.
Immerse the clams in salty water to remove the sand, then rinse under cold running water. Set aside to drain. Sweat shallot and garlic in oil until soft, about 3 minutes. Add the clams and white wine. Cover and steam (à la marinière) for 3 to 5 minutes until the clams open. Remove clams from the cooking liquid, allow to cool and loosen from shells. Reserve until needed.
Parsley and garlic sauce:
Reduce oven temperature to 160°C. Roast garlic cloves in foil until soft, about 45 minutes. Remove from oven and cool. Sweat shallot in butter until soft, about 3 minutes. Deglaze with wine and reduce by 50% and add fish stock and reduce by 50%. Add cream, strain through a fine mesh sieve and season. Pass garlic through a sieve and add to sauce a little at a time whilst tasting. Keep warm.
Blanch samphire in salted boiling water for about 2 minutes. Refresh by placing into iced water and set aside until needed. Increase oven temperature to 180°C. Put olive oil into a large oven proof frying pan over a high heat and place cod skin side down to color. Add chopped butter and allow to foam. Remove from heat and place in oven for 5 minutes. Remove and baste with butter and keep warm.
Aerate sauce (optional) using a hand blender until frothy, add chopped parsley and clams. Reheat the gnocchi and samphire separately in boiling water. Pour a little sauce into the center of a plate, 4 with the pan roasted cod. Arrange gnocchi and clams around the cod and finish with samphire.