Pat the steak cubes thoroughly dry with paper towels. Season evenly with kosher salt, cracked black pepper, onion powder, and smoked paprika. Let sit at room temperature for 10 minutes.

Place halved baby potatoes in a microwave-safe bowl with 1 tablespoon of water. Cover and microwave on high for 3 to 4 minutes until fork-tender but not mushy. Drain and pat dry.

Heat 1 tablespoon of olive oil and 1 tablespoon of butter in a large cast iron skillet over medium-high heat. Add the par-cooked potatoes and sear undisturbed for 3 to 4 minutes until golden and crisp-edged. Transfer potatoes to a plate.

Increase heat to high and add the remaining tablespoon of olive oil. Add the seasoned steak cubes in a single uncrowded layer. Sear undisturbed for 2 minutes to develop a deep caramelized crust, then flip and sear for 1 to 2 minutes more for medium-rare.

Reduce heat to medium-low. Return the potatoes to the skillet and drop in the remaining 3 tablespoons of butter, minced garlic, and chopped rosemary. Once the butter foams, spoon the fragrant garlic herb butter continuously over the steak and potatoes for 1 minute.

Remove from heat immediately to prevent overcooking. Garnish with chopped fresh parsley and serve hot directly from the skillet or in warm shallow bowls.