In a mixing bowl, toss the raw shrimp with the Cajun seasoning and olive oil until evenly coated.

Add the kosher salt to a large pot of water and set it over high heat to bring to a boil. While that water comes up to temperature, preheat a wide skillet over medium-high heat.

Drop the penne into the boiling salted water and cook to al dente. Scoop out at least 1/2 cup of the starchy pasta water before draining, then drain the pasta and set it aside.

Right after adding the pasta to the water, melt the butter in the hot skillet. Lay the seasoned shrimp in a single layer and sear until golden brown on both sides, flipping just once; work in two batches if necessary to avoid crowding. Transfer the cooked shrimp to a plate, then lower the skillet heat to medium.

Pour the heavy cream into the same skillet, using a flat wooden spatula to scrape up any browned bits stuck to the bottom. Let the cream bubble around the edges, then turn the heat to low and stir in the Parmesan until it melts into a smooth sauce.

Return the pasta and shrimp to the skillet and toss to coat evenly. If the sauce is too thick, add the reserved pasta water one tablespoon at a time until it reaches the consistency you like. Taste and season with extra salt if needed.