Pat steak cubes dry and season with salt and pepper; let rest at room temperature for 20–30 minutes.
Heat olive oil in a large cast-iron skillet over high heat and sear steak bites in a single layer for 2–3 minutes per side. Work in batches and transfer cooked steak to a plate, loosely covering it.
Reduce heat to medium and add 2 tablespoons butter and 4 minced garlic cloves to the skillet. Stir until fragrant, about 30 seconds.
Return steak bites to the skillet and toss in garlic butter, then remove again to avoid overcooking.
Melt remaining 2 tablespoons butter in the skillet and sauté 2 minced garlic cloves for 30 seconds.
Pour in heavy cream and simmer gently for 2–3 minutes until slightly thickened.
Stir in parmesan cheese, black pepper, crushed red pepper flakes, and salt to taste until smooth.
Return steak bites to the skillet, coat with the parmesan cream sauce, and heat for 1–2 minutes.
Garnish with chopped parsley and serve immediately with preferred sides.