Season the buttermilk with salt, pepper and Old Bay seasoning. Place crabs in the buttermilk mixture. Let the crabs marinate covered and chilled, for at least 30 minutes and up to 2 hours. Remove and pat dry. In a bowl, mix together the flour, cayenne and salt and pepper to taste. Dredge the crab in the flour, shaking off excess. Choose a heavy frying pan (or pans) that will hold all the crabs in one layer without crowding. Place the pans over moderately high heat and add butter and oil to create a thin layer (about 1/4-inch) in each pan. After the foam subsides, add the floured crabs. Cook for 2 to 3 minutes, or until the crabs are golden brown on the underside. Turn the crabs over. Cook for 2 to 3 minutes, or until the other side is golden brown. Transfer them to paper towels to drain briefly.
Have ready English muffins, split in half and flattened with a rolling pin. In a small saucepan combine unsalted butter, lemon juice and parsley, shallot, and salt and pepper to taste, heat the mixture over moderately low heat, stirring until the butter is just melted and reserve it. Brush muffins with herbed butter and place on griddle. Heat for 3 to 5 minutes, remove and place 1 crab between 2 muffin halves.