First, prepare the cilantro-chili butter. Place all the ingredients in a bowl along with a generous seasoning of salt and pepper. Beat with a fork until well combined. Spoon the butter along the length of a large sheet of plastic wrap and roll up in the wrap to shape the butter into a log. Holding the sides of the wrap, roll the log on the work surface to even out the thickness. Refrigerate for a few hours until firm.

Soak 12 bamboo skewers in cold water for about 20 minutes. This will help to prevent them from burning too quickly during grilling.

Mix the oiive oil, chopped herbs, and lemon zest and juice together in a bowl. Thread two scallops and two shrimp alternately on each soaked skewer, then brush with the lemon and herb marinade. Place the skewers on a tray, cover with plastic wrap, and refrigerate for 20-30 minutes.

Prepare the barbecue or heat a cast-iron grill pan until hot. Season the scallops and shrimp with salt and pepper. Grill over coals or on the hot pan until the scallops and shrimp are opaque and lightly charred, 1½-2 minutes on each side. During cooking, baste occasionally with any leftover marinade. Remove to a serving plate and top with slices of the cilantro-chiii butter. Serve immediately.