1. In a small skillet, toast the cumin seeds over moderate heat until fragrant, about 1 minute. Let cool, then transfer to a mini-processor or mortar. Add the garlic cloves and process or crush to a coarse paste. Scrape the mixture into a bowl and stir in 1/3 cup of the lemon juice, the bay leaves, paprika, chile powder, oil and 1 tablespoon of kosher salt.
2. Pour 1 inch of water into 2 very large stockpots. In each pot arrange a round rack or steamer basket; it should just fit inside. Cover and bring to a boil over high heat. Using tongs, pick up a crab from the back and spread about 2 tablespoons of the paste over the back and on the front claws. Set the crab on the rack in 1 of the pots and repeat with the remaining paste and crabs, adding 10 crabs to each pot. Cover the pots and steam until the crabs are bright pink all over, about 15 minutes.
3. Meanwhile, in a small saucepan, melt the butter over low heat. Remove from the heat and stir in the scallions, cilantro, lemon zest and the remaining 1/3 cup of lemon juice. Season with salt; keep warm.
4. Pour the butter into small dipping bowls. Using tongs, transfer the crabs to the table. To eat, begin by pulling off a crab's top shell. Break off the claws and gently tap them with the mallet to crack the shells without crushing the meat. Remove the meat from the shells with your fingers. Break the body into sections and remove the meat with your fingers.