Dry brine (recommended): Salt the steak and refrigerate uncovered on a rack for 2 hours or up to overnight.
Bring to temp: Set steak out at room temperature for 45 to 60 minutes. Pat dry.
Season: Mix pepper, paprika, chili flakes, and cayenne. Rub steak with oil, then press seasoning onto all sides.
Slow cook: Heat oven to 250°F. Bake steak on a rack until it reaches 110°F for medium-rare (or 120°F for medium), about 45 to 75 minutes depending on thickness.
Make garlic butter: Stir together butter, garlic, chili flakes, paprika, Dijon, Worcestershire, parsley, and salt. Set aside.
Sear: Heat a cast iron skillet over high heat until very hot. Add a little oil. Sear steak 60 to 90 seconds per side, plus edges, until browned.
Rest: Rest 10 to 15 minutes. Top with garlic butter while resting.
Slice: Remove ribeye from the bone, then slice against the grain. Spoon melted butter and juices over the top before serving.