How to Make It
In a large pot, bring the stock to a boil with the shrimp shells and corn cobs. Cover and simmer for 30 minutes. Remove the cobs. Strain the stock through a fine sieve into a large, heatproof bowl, pressing on the shells to remove as much stock as possible. Wipe out the pot.
In a small skillet, toast the celery and fennel seeds over moderately high heat until fragrant and starting to pop, about 45 seconds. Let cool completely. In a spice grinder or mortar, grind or crush the seeds to a powder.
In the large pot, melt the butter over moderately high heat. Add the yellow pepper, onion, celery and chile and cook until they begin to soften, about 2 minutes. Add the ground fennel and celery seeds and cook for 1 minute. Add the potatoes and shrimp stock and bring to a boil. Cover and simmer until the potatoes are tender, about 8 minutes. Remove from the heat.
Stir the shrimp and crabmeat into the chowder. Cover and let stand until the shrimp turn pink on the outside, about 3 minutes. Stir in the heavy cream and corn kernels. Bring to a simmer over moderate heat; immediately reduce the heat to low and simmer gently until the shrimp are white throughout, about 3 minutes. Ladle the chowder into bowls and garnish with chives. Serve with buttered toast.