Clean and gut the trout. Wash under cold water and pat dry on kitchen paper. Take a piece of aluminium foil that is at least the length of the fish plus 6" (15cm) and lay it on a flat surface. Using your fingers, smear a little butter over the foil and then lay the trout on top. Wrap the foil into an envelope around the fish.
Grill under a preheated moderate grill for about 6 minutes, then turn the package over and grill the other side for a further 4 minutes. Remove the foil and serve immediately.