To prepare, build a fire in your grill; if using a gas grill, turn both burners to high.
Put all spice paste ingredients in a food processor and puree. Rub steaks all over with paste (it will be rather thick) and set aside.
Combine ingredients for cilantro-lime butter in a small bowl and mash with a fork until well mixed and an even consistency. (May be made earlier in the day.)
When all coals are covered with gray ash and fire is medium hot (you can hold your hand 5 inches above grill for 3 to 4 seconds), put steaks on grill directly over coals and cook until well seared, about 7 minutes. Turn and cook another 7 to 10 minutes for medium rare. Remove from heat, top each steak with a couple tablespoons of butter and serve.