Season the crabs with salt and pepper and place in the buttermilk. Soak for 1 hour. Remove and pat dry.
While the crabs are soaking, prepare the mojo: heat the olive oil in a small, heavy saucepan over a moderately low flame. Add the garlic and cook slowly for 5 minutes, or until golden, not brown. Add the lime juice, stir, and bring to a boil. Remove from heat, season with salt and pepper, and add cilantro. Refrigerate until cool.
To prepare the crabs: In a bowl, mix together the flour, cayenne and salt and pepper to taste. Dredge the crabs in the flour, shaking off the 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 enough butter to create a thin layer (about 1/4 inch) in each pan. After the foam subsides, add the floured crabs and weight them (with bricks or anything you can improvise). Cook for 2 to 3 minutes, or until the crabs are golden brown on the underside. Turn the crabs over, and weight them again. Cook for 2 to 3 more minutes, or until the other side is golden brown. Transfer them to paper towels to drain briefly, dab each crab with some of the mojo and serve.
Courtesy of Taste, by David Rosengarten