Preheat oven to 375°F Spray or butter a 9 x 13" baking dish.
To make macaroni: Add bouillon to large pot of water and bring to boil. Once water is boiling, add macaroni and cook until al dente (not quite fully cooked), usually about 4-5 minutes. Drain macaroni in colander.
To make sauce: While the water is heating and pasta is cooking, melt butter In a large saucepan, over medium heat. Once melted, whisk in flour and cook for about 1 minute, until the flour starts to brown a bit. While whisking constantly, add milk and then cream in a steady stream. Allow the sauce to come to a simmer but keep whisking so it doesn't burn!
Once the sauce comes to a simmer, stir in the 2 cups Jack cheese and 2 cups cheddar (don't use the cheese for the topping!), adding about a cup at a time. Stir constantly making sure each addition of cheese is melted before adding the next. Add 2 tsps Cajun seasoning, dried parsley, salt and pepper to taste and remove from heat.
Add the cooked, drained pasta to cheese sauce. Stir until macaroni is fully coated. Pour the cheesy mixture into the prepared casserole dish.
Prepare the Topping: In a small bowl, mix the melted butter with the Panko bread crumbs, 1/2 tsp Cajun seasoning and black pepper, mixing with a fork until bread crumbs are evenly coated.
Sprinkle the macaroni and cheese with the remaining 2 cups shredded cheeses. Sprinkle the bread crumb mixture on top of the cheese.
Bake until browned and bubbly, about 25 to 30 minutes. Allow to sit for a few minutes before serving. Hot stuff!