Fill a large pan with water. Add kosher salt, and bring to a boil over high heat. Add crawfish, and cook about 1 minute. Transfer crawfish to a large ice bath.
With a sharp knife, cut through shell on belly side the entire length of crawfish. Remove vein, and remove sack behind eyes in head. Rinse crawfish thoroughly, removing any remaining sediment.
In a medium bowl, combine butter and Cajun seasoning, and beat at medium-high speed with a mixer until smooth and creamy. Cover, and refrigerate up to 3 days.
Spray grill basket with nonstick nonflammable cooking spray. Preheat grill to medium-high heat (350° to 400°). Place crawfish in grill basket, and grill, shaking basket occasionally, until shell turns bright red and meat is cooked through, about 6 minutes. In a small saucepan, melt butter mixture over medium-high heat. Serve melted butter with crawfish.