Start by patting the steak dry with paper towels on both sides. Season it with salt, pepper, and garlic powder. Let it sit at room temperature for 15 to 20 minutes. Dry the shrimp and lightly season with salt, pepper, and smoked paprika. Mince the garlic, grate the Parmesan cheese, and measure out your sauce ingredients.
Boil a large pot of water and add salt until it tastes like the sea. Cook the bucatini according to the package instructions, usually 9 to 11 minutes, until it’s al dente (slightly firm). Save at least ½ cup of the pasta water before draining the pasta. Do not rinse, as the starch helps the sauce stick.
Heat a cast-iron skillet over high heat for about 2 minutes. Add one tablespoon of olive oil; it should shimmer. Place the steak in the skillet without moving it. Sear for 3 to 4 minutes on each side for medium-rare. The internal temperature should be 130 to 135°F for medium-rare, or 140 to 145°F for medium.
After cooking, move the steak to a cutting board and let it rest for 5 to 7 minutes. Once rested, slice it thinly against the grain.
In the same skillet (wipe it out if needed), melt 1 tablespoon of butter over medium-high heat. Add the shrimp in a single layer and cook for 1 to 2 minutes on each side until they turn pink and curl slightly. Remove them from the heat to avoid overcooking. Set the shrimp aside.
In a large skillet, melt 2 tablespoons of butter over medium heat. Add minced garlic and cook for 60 seconds until fragrant. If you are using chicken broth, add it now and let it reduce for 1 minute. Stir in the Calabrian chili paste and cook for another 30 seconds.
Pour in heavy cream and simmer for 3 to 4 minutes until it thickens slightly. Add lemon juice and mix in. Lower the heat and gradually add the Parmesan cheese until smooth. Use pasta water to create a silky consistency.
Add the drained bucatini to the sauce and toss over low heat for 1 to 2 minutes until it absorbs the sauce. If it is too thick, add some pasta water. Serve in bowls, topping with sliced steak, shrimp, fresh Parmesan, lemon zest, and parsley or basil.