Bring a large pot of salted water to a boil. Cook shell pasta according to package directions until al dente. Reserve 1/2 cup pasta water before draining.

While pasta cooks, pat steak cubes dry with paper towels and season generously with salt and pepper.

Heat olive oil in a large cast-iron skillet over high heat until shimmering.

Add steak bites in a single layer (work in batches if needed). Sear for 2 minutes without moving, then flip and cook 1-2 minutes more.

Reduce heat to medium. Add 2 tbsp butter and minced garlic. Toss steak in the garlic butter for 1 minute. Remove steak from pan and set aside.

In the same skillet, reduce heat to low. Add 2 tbsp butter and garlic, sauté briefly.

Pour in heavy cream and scrape up any browned bits from the bottom. Simmer for 3-4 minutes until slightly thickened.

Stir in parmesan cheese until melted. If sauce is too thick, add reserved pasta water to thin it out.

Add the drained pasta to the sauce and toss to coat. Top with the steak bites and garnish with parsley.