Pat the steak cubes completely dry with paper towels. Season generously with salt and black pepper on all sides. Let sit at room temperature for 5 minutes.

Heat a large cast iron skillet or heavy-bottomed pan over high heat. Add olive oil and 1 tablespoon of butter, swirling to coat the pan.

Once the pan is smoking hot, add the steak cubes in a single layer without overcrowding. Sear for 2 minutes without moving, then flip and cook for another 1-2 minutes for medium-rare. Work in batches if needed.

Transfer the seared steak bites to a plate and tent loosely with foil. Let rest while you prepare the garlic butter.

Reduce heat to medium-low. Add remaining 3 tablespoons of butter to the same skillet. Once melted, add minced garlic and red pepper flakes if using. Cook for 30-60 seconds until fragrant, stirring constantly to prevent burning.

Return the steak bites and any accumulated juices to the skillet. Toss quickly to coat in the garlic butter sauce, about 30 seconds.

Transfer to a serving platter and garnish with fresh chopped parsley. Serve immediately while hot.