Place the shrimp shells and seafood stock in a saucepan and simmer for 15 minutes. Strain and reserve the stock.
Meanwhile, heat olive oil in a large pot or Dutch oven. Add the leeks, celery and carrots and cook them for 10 minutes over medium low heat until tender. Add garlic and cook 1 more minute.
Chop shrimp into small pieces. Add the cayenne pepper and shrimp and cook over medium heat for 3 minutes, stirring occasionally. Add the Cognac and cook 1 minute, then add the sherry and cook 3 more minutes. Transfer mixture to a blender or food processor and puree.
In the same pot, melt the butter. Add 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 shrimp puree, stock, tomato paste, salt, pepper and basil. Gently heat until hot but not boiling. Season to taste and serve.