To butterfly the hens, turn them breast side down and using poultry shears, cut down each side of the backbone and discard it. Turn the birds over and open them out flat, pressing down hard on the breastbone. Thread 2 skewers diagonally through each hen from the wings to the thigh bones.
To make the marinade, skewer the chili peppers and garlic together and cook on a preheated medium-hot outdoor grill or under a broiler for 10 minutes, turning frequently until evenly browned. Scrape off and discard the skins from the peppers and chop the flesh coarsely. Put the flesh (and seed if you like the heat) into a blender. Add garlic and all the remaining marinade ingredients and blend to a puree. Pour the marinade over the hens and let marinate in the refrigerator overnight. Retun them to room temperature for 1 hour before cooking.
When ready to cook, remove the birds from their marinade and grill over preheated medium-hot coals for 12 minutes on each side (or until juices run clear), basting occasionally. Remove from the heat and let rest for 5 minutes, then serve with corn salsa.