Preheat oven to 350°F. Using a pastry brush, in a 4-quart casserole dish, butter the bottom and up the sides of the dish with the 2 tablespoons of room temperature butter. Set aside.
In a large stockpot, boil your cavatappi noodles for 8 minutes. Once cooked, drain them and return them to your stockpot. While your noodles are boiling, start your cheese sauce.
In a large saucepan, on medium-high heat, melt your remaining ¼ cup of butter. Add the flour and whisk until all the flour has been incorporated with the melted butter and gets light brown in color.
While whisking your butter and flour, slowly pour in your whole milk and half & half. Make sure you whisk constantly to ensure that you do not have any lumps in your cheese sauce.
Add your Dijon mustard, season salt, dried parsley, and black pepper to your milk mixture. Continue to cook your sauce for 6 to 8 minutes and whisk to make sure that you do not burn your milk mixture.
Once the milk mixture has thickened, turn off the heat and add your shredded sharp yellow cheese and shredded Monterey jack cheese. You will not want to add your cheese all at once, but in 1 cup portions at a time so that as you whisk it in, you are creating a smooth cheese sauce.
Once all your sharp yellow cheddar and Monterey jack cheese is incorporated and your sauce is smooth, pour it into your stockpot with your cook and drained noodles. Stir to coat evenly.
Pour half of your cheesy noodles into your prepared casserole dish. Sprinkle, in an even layer, 1 ½ cups of your shredded gruyère cheese.
Dollop your sour cream over the top of your shredded gruyère cheese and smooth it out into an even layer.
Add the remaining cheesy noodles to your casserole dish and top evenly with the remaining ½ cup of shredded gruyère cheese.
In a medium-sized bowl, combine all of your topping ingredients so that they are mixed well. Evenly sprinkle this topping mixture over your mac & cheese mixture.
Bake your million dollar mac and cheese for 30 minutes or until it is golden brown and the cheese sauce is bubbly.
Once baked, allow your million dollar mac and cheese to rest on the counter for 10 to 5 minutes before serving.