Transfer half of the langoustine claws to the bowl of an electric mixer fitted with a paddle and begin to mix on low speed for 1 minute to break them into smaller chunks. Add the remaining claws and continue mixing for another minute, then increase the speed to medium and mix until they break down into a mash. Add the remaining ½ tablespoon salt, the ginger, lemongrass, Szechuan peppercorns, fennel seeds, black peppercorns, coriander seeds, and tarragon and mix until well combined. Transfer the mixture to a medium saucepan and cover with 1 quart water. Gently stir a few times, then place over medium heat. Bring to a gentle simmer, undisturbed, allowing the mixture to form a solid mass, or raft. Once the raft begins to rise to the surface, gently poke a hole in the top. Continue to gently simmer for 20 minutes, occasionally basting the raft by ladling broth from the hole. Carefully strain the liquid by ladling through a fine-meshed sieve lined with 3 layers of wet cheesecloth. Reserve the liquid, chilled.