In a large bowl, combine the crayfish, mayonnaise, crème fraîche, dill, and lemon juice. Season the mixture with salt and pepper, and stir together with a wooden spoon to thoroughly mix.
Melt the butter in a wide skillet and lightly toast the brioche buns, crust side up. Try to get the buns lightly golden without being too soggy.
Spoon the crayfish mixture into the buns and serve immediately.