For the cowboy butter: Well in advance, make the cowboy butter. Place the softened butter in the bowl of a mixer fitted with a paddle attachment. Whip the butter until it's fluffy. Add the parsley, red pepper flakes, black pepper, garlic, lemon zest and the juice of half the lemon. Mix it until it's totally combined, scraping the sides as needed. Add more parsley, more lemon juice, more pepper or salt if you'd like.
Lay out a long piece of plastic wrap and scoop the butter mixture in a long strip down the middle of it. Carefully pull one side of the plastic wrap over the butter, squeezing it gently to form it into a log. Continue to roll the log of butter into a roll. When it's all rolled up, twist the ends (like a piece of candy) until they become very taut (this means the butter is pressing together inside the plastic to form a cohesive roll). Place the roll of butter into the fridge so it will harden, or into the freezer if you need to speed along the process.
For the steaks: Preheat the oven to 475 degrees F. Sprinkle salt and pepper on both sides of the steaks.
Melt the butter in a heavy ovenproof skillet over medium-high heat. When it's melted and golden brown, sear the steaks for about 45 seconds per side. Set in the oven to finish, about 3 minutes.
Lay a thick slice of cowboy butter on top of each steak so it will begin to melt. Serve within 5 minutes.
Other uses for cowboy butter: Stir into cooked pasta, lay a pat on grilled chicken or fish, stir it into rice or spread on crusty French bread.