Peel shrimp and devein, leaving tails on and reserving the heads and shells. Refrigerate shrimp.
In a small Dutch oven, add shrimp heads and shells, and cover with 7 cups cold water. Bring to a boil over medium-high heat. Reduce heat to medium, and cook, stirring occasionally, for 15 minutes. Skim any froth as it rises to the surface. Strain through a fine-mesh sieve into a bowl, and set aside. Reserve 1 cup shrimp stock. Remaining stock can be refrigerated up to 1 week or frozen up to 3 months.
In a large skillet, melt 5 Tbsp. butter over high heat. Add rosemary, pepper, Cajun seasoning, shallot, and garlic. Cook, stirring constantly, until fragrant, about 1 minute. Add reserved stock, Worcestershire, hot sauce, and lemon juice. Add shrimp, and cook just until pink and firm.
Add beer, and cook 2 to 3 minutes more. Decrease heat to low, and add remaining 7 Tbsp. butter. Gently stir as the butter melts into the sauce and the sauce is emulsified. Serve immediately with French bread.