Pat the steak dry. Season with kosher salt on both sides. For best results, refrigerate uncovered 8–24 hours, then bring to cool room temperature for 30–45 minutes before cooking.
Mix pepper, garlic powder, paprika, chili powder, onion powder, cayenne, and brown sugar (optional). Rub steak with oil, then press seasoning blend onto both sides.
Preheat oven to 250°F. Place steak on a wire rack over a baking sheet. Cook until internal temp reaches 110°F (rare), 115°F (medium-rare), or 125°F (medium).
Rest steak 10 minutes while heating a cast iron skillet over high heat. Add a small drizzle of oil.
Sear steak 1–2 minutes per side until deeply browned. Sear edges 30–45 seconds.
Lower heat to medium. Add butter. When melted and foamy, add minced garlic and red pepper flakes. Baste steak for 30–60 seconds. Turn off heat, stir in parsley and lemon juice if using.
Rest steak 10–15 minutes. Remove meat from bone, slice against the grain, and spoon spicy garlic butter over the slices.