How to Make It
Preheat the broiler on the lowest setting and position a rack 12 inches from the heat. Rub 1 tablespoon of the butter on a small rimmed baking sheet.
In a large bowl, whisk the egg with the mayonnaise, lemon juice, salt, black pepper and cayenne. Gently fold in the crabmeat and 1/4 cup of the bread crumbs, breaking up the crab into smaller pieces. Refrigerate for 10 minutes.
Pack one-fourth of the crab mixture into a 1/2-cup dry measuring cup and turn out onto the prepared baking sheet. With slightly damp hands, gently form it into a patty. Repeat with the remaining crab mixture; the patties will be very loose. Sprinkle the remaining 1/4 cup of bread crumbs on top of the patties.
Broil the crab cakes for 5 minutes, or until the crumbs are golden. Drizzle the remaining 4 tablespoons of butter on top and broil for 1 minute longer. With a large spatula, carefully transfer the crab cakes to plates. Serve with lemon wedges.