Preheat oven to 400°F. Measure out the half and half, and set aside. Lightly spray a deep 9" x 13" baking dish, and set aside.

Cook pasta per package directions, to slightly less than al dente. Drain very well; set aside.

In a Dutch oven or large skillet over medium-low heat, add 4 tablespoons of the butter and melt. Add garlic powder, onion powder, cayenne pepper, salt, and pepper to the butter. Stirring constantly, “toast” the spices in the butter for 30 seconds, until fragrant.

Add flour to the pan and whisk to combine (a flat whisk works great for this!). Cook for 2 minutes, stirring fairly constantly. Slowly stream in half and half, whisking constantly, until lump-free. Turn up heat just a bit, between medium and medium-high. Bring mixture to a simmer and simmer until thickened, stirring occasionally, about 5 minutes.

In a large bowl, mix together gruyere cheese and white cheddar cheese. Remove Dutch oven from heat and add half of the mixed cheese. Fold until cheese is melted.

Add shrimp and cooked pasta to the Dutch oven, and fold to combine with the sauce. Transfer mixture to prepared baking dish.

In a small bowl, melt remaining 2 tablespoons of butter. Add breadcrumbs and grated Parmesan, and stir to incorporate evenly.

Add remaining cheese to the top of the dish. Sprinkle breadcrumbs evenly over the top. Bake for 20-25 minutes, or until bubbly and the top is golden brown. If desired, place dish under broiler for a couple minutes to toast the breadcrumbs even more. Remove from oven and let sit for about 10 minutes before serving. Sprinkle parsley over the top, right before serving, if desired.