Peel the shrimp.  Combine the shrimp shells and the seafood stock in a large pot and simmer for 15 minutes. Strain and reserve the stock. Heat the oil in a large pot over medium-low heat.  Add the leeks and cook them for 10 minutes, stirring occasionally.  Stir in the garlic and cook for 1 minute.  Stir in the cayenne pepper and shrimp and cook for 3 minutes.  Stir in the Cognac and cook for 1 minute.  Stir in the sherry and cook for 3 minutes.  Remove to a food processor and lightly process until the shrimp and leeks are coarsely pureed.  In the same pot, melt the butter over medium heat until the foaming subsides.  Add the flour and whisk until the foaming again subsides.  Add the half-and-half and whisk until thickened (about 3 minutes).  Add the stock and whisk until thickened (about 3 minutes).  Stir in the tomato paste, pureed shrimp, salt and pepper and heat through over low heat (do not boil).  Season to taste and serve.