How to Make It
In a bowl, mix all of the ingredients except the steaks. Lay the steaks in a large roasting pan in a single layer. Pour the marinade over the meat and turn to coat. Cover and refrigerate for 2 hours.
In a medium skillet, heat the olive oil. Add the shallots and cook over moderate heat until softened and lightly browned, about 10 minutes. Add the soy sauce and simmer until evaporated, 2 minutes; let cool.
In a bowl, mix the shallots, shiso leaves and peppercorns into the butter. Transfer to the refrigerator.
Light a grill. Grill the steaks over a hot fire for 3 to 4 minutes per side for medium-rare meat. Transfer to a carving board and let rest for 5 to 10 minutes. Meanwhile, grill the scallions, turning, until lightly charred all over, about 2 minutes. Thinly slice the steaks against the grain and transfer to a platter along with the scallions. Top the skirt steaks with the shiso butter and serve them at once.