For the marinade, heat the oil in a small saucepan over medium heat until very hot but not boiling. Drop in a red pepper flake; if it skips to the surface of the oil and bubbles, the oil is the correct temperature. If the pepper turns brown or black, the oil is too hot and you will have to let it cool a while. If the pepper flake sinks and does not bubble, the oil is not hot enough. Keep heating and testing until you’ve got it right. When the oil is the correct temperature, remove it from the heat and add the remaining pepper flakes. They will sizzle and bubble on the surface of the oil a few minutes. When the action subsides and the oil turns a pale orange color, add the lemon juice, garlic, and ground pepper. Let cool completely.
To butterfly the poussins, insert a heavy, sharp knife at the neck cavity and carefully cut down one side of the backbone. Cut along the other side of the backbone to separate it from the body. Pull open the sides of the poussin, turn it skin side up and press firmly on the breast with the heel of your hand to flatten it. Fold the wings back against themselves so they lie flat. Trim the excess skin from the neck end. Place the poussins in a shallow nonaluminum container; cover with the marinade and turn the poussins to coat. Cover and refrigerate overnight.
For the butter, heat the butter with the garlic in a small saucepan over low heat until melted. Heat the olive oil until very hot (see instructions for heating in the marinade) in a small saucepan over medium heat. Remove it from the heat and add the red pepper flakes, cool for 10 minutes, then the garlic butter, then the lemon juice and salt.
Let the poussins warm to room temperature. Heat the grill or broiler. Remove the poussins from the marinade and sprinkle lightly with salt. Grill or broil skin side away from the flame 5 to 6 minutes, basting once or twice with the pirl-piri butter. Turn and cook until the juices run clear when the leg is pierced with a skewer and the skin is golden and slightly crisp, about 5 minutes more.
Serve the poussins with the remaining butter hot in ramekins and accompany with fried potatoes and sliced tomatoes sprinkled with a little chopped mint or cilantro.