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.