Pat the steak cubes dry thoroughly with paper towels, then season all sides with 0.75 tsp salt and 0.5 tsp black pepper. Let sit at room temperature while you prepare the pan.

Heat a large cast-iron or heavy skillet over high heat until very hot, then add the olive oil.

Add the steak bites in a single layer without crowding and sear for 2–3 minutes per side until browned; work in batches if needed. Transfer seared steak bites to a plate and cover loosely.

Reduce heat to medium and add 2 tbsp butter and 4 minced garlic cloves to the same skillet. Stir for 30 seconds, scraping up browned bits, until the garlic is fragrant.

Return the steak bites to the skillet, toss in the garlic butter for 30–60 seconds, then transfer the steak and any juices back to the plate to prevent overcooking.

In the same skillet, add the remaining 2 tbsp butter and the remaining 2 cloves minced garlic. Sauté for 30 seconds over medium heat.

Pour in the heavy cream, stirring constantly, and let it simmer for 2–3 minutes until slightly thickened and bubbling gently.

Stir in the grated Parmesan, remaining 0.25 tsp black pepper, and red pepper flakes if using. Cook, stirring, until the sauce is smooth, then taste and add the remaining 0.25 tsp salt if needed.

Return the steak bites and any accumulated juices to the skillet and toss in the Parmesan cream sauce for 1–2 minutes, just until heated through and coated.

Remove from heat, sprinkle with chopped parsley, and serve immediately.