Place the shrimp shells and seafood stock in a saucepan and simmer for 15 minutes. Strain and reserve the stock. Add enough water to make 3 3/4 cups. Meanwhile, heat the olive oil in a large pot or Dutch oven. Add the leeks and cook them for 10 minutes over medium-low heat, or until the leeks are tender but not browned. Add the garlic and cook 1 more minute. Add the cayenne pepper and shrimp and cook over medium to low heat for 3 minutes, stirring occasionally. Add the Cognac and cook for 1 minute, then the sherry and cook for 3 minutes longer. Transfer the shrimp and leeks to a food processor fitted with a steel blade and process until coarsely pureed. In the same pot, melt the butter. Add the flour and cook over medium-low heat for 1 minute, stirring with a wooden spoon. Add the half-and-half and cook, stirring with a whisk, until thickened, about 3 minutes. Stir in the pureed shrimp, the stock, tomato paste, salt, and pepper and heat gently until hot but not boiling. Season, to taste, and serve hot. ;