Fill a large pot with 6 cups of cold water and add 1 tablespoon of kosher salt. Bring it to a rolling boil—this is important because the pasta needs hot water to cook evenly. While your water heats, pat your shrimp completely dry with paper towels. This step makes an enormous difference in browning. Wet shrimp will steam instead of sear, and you'll lose that caramelized exterior that makes this dish special. Mince your garlic if using fresh, and have your Cajun seasoning measured and ready. When cooking, you want everything prepped before you start the actual cooking process.

Add your penne to the boiling salted water when the water is at a rolling boil. Stir immediately to prevent sticking. Cook according to package directions, usually 9-11 minutes, but taste it at the 8-minute mark. You want it tender but still with a slight firmness when you bite it—this is al dente. Shrimp cooks so quickly that your pasta needs to be ready at the same time. About 1 minute before your pasta finishes, reserve 1 cup of the pasta cooking water in a small bowl or measuring cup. This starchy water is your secret weapon for creating a silky sauce that clings to the pasta. It's the difference between sauce that coats and sauce that pools at the bottom.

While your pasta cooks, heat a large skillet over medium-high heat. Add your olive oil and let it shimmer—you should see it move easily around the pan. Add the dry shrimp in a single layer and don't move them for 2-3 minutes. This creates that beautiful golden sear. Flip them and sear the other side for another 1-2 minutes until the shrimp is mostly opaque with just a tiny hint of translucence in the center. Sprinkle the Cajun seasoning over the shrimp in the last 30 seconds of cooking and toss quickly to coat. The heat will bloom the spices and release their flavor. Don't overcook—shrimp goes from perfect to tough very quickly, usually in about 4-5 minutes total.

Lower the heat to medium. Add the butter to the pan with the shrimp. Once it melts, pour in your heavy cream. Stir gently and let it warm through for about 2 minutes, but don't let it boil vigorously. Heavy cream can break if it gets too hot too fast, which means it separates and becomes grainy. You want gentle heat and slow integration. Add your minced garlic (if using fresh) and stir. Taste the sauce and add salt and pepper to your preference. Remove the pan from heat before adding the Parmesan cheese. The residual heat will melt it evenly without making it grainy.

Drain your pasta in a colander—don't rinse it. You want that starch on the noodles. Add the hot pasta directly to the pan with the sauce and shrimp. Pour in about ½ cup of your reserved pasta water and toss everything together gently for about 1 minute. The starch from the pasta water will emulsify with the cream, creating a silky sauce that coats every noodle. The pasta will continue to absorb liquid as you plate and serve, so if it looks slightly loose in the pan, that's exactly right. Serve immediately in warm bowls with extra Parmesan on the side and a crack of fresh black pepper.