How to Make It
Toast ancho chiles in a small skillet over medium, turning often, until they start to blister, about 4 minutes. Transfer chiles to a bowl and cover with boiling water. Let stand 15 minutes. Drain well and finely chop.
Mash softened butter with chiles, chives, zest, 2 teaspoons salt, crushed red pepper, and 1/2 teaspoon black pepper. Roll butter in parchment; freeze until firm.
Preheat oven to 425°F. Season roast all over with salt and pepper. Heat oil until shimmering in a large cast-iron skillet over medium-high. Add roast and cook, turning occasionally, until browned all over, about 8 minutes. Transfer skillet to oven, and roast until an instant-read thermometer inserted in thickest portion of meat registers 120°F, 15 to 20 minutes. Transfer roast to a carving board, tent with aluminum foil, and let rest 10 minutes.
Slice the meat against the grain and transfer to a platter. Using a box or hand grater, grate some compound butter over meat, and serve immediately.