While the tomatoes are cooking, heat a large pot over medium-high heat. Add butter and melt until foaming. Add shallots and cook until soft, approximately 5 minutes. Add tomato paste, flour, cayenne, and salt, stirring constantly for approximately one minute. Whisk in chicken broth and bring to a simmer, ensuring all of the flour mixture is incorporated into the broth. Once the broth begins to thicken, add the reserved tomato juice and bring to a slow boil.