Cook the shell pasta according to package instructions until al dente. Drain and set aside, reserving 1/2 cup of pasta water.
In a large skillet over medium-high heat, melt 2 tablespoons of butter. Add the steak bites, season with salt, black pepper, and red pepper flakes (if using). Sear the steak for about 3-4 minutes until browned and cooked to your desired doneness. Remove the steak from the skillet and set aside.
In the same skillet, add the remaining 2 tablespoons of butter and minced garlic. Sauté for about 1 minute until fragrant.
Pour in the heavy cream and bring to a simmer. Stir in the Parmesan cheese until melted and smooth. If the sauce is too thick, add reserved pasta water a little at a time until you reach your desired consistency.
Add the cooked shell pasta and steak bites back into the skillet. Toss to coat everything in the creamy sauce.
Serve immediately, garnished with fresh parsley.