Preheat your oven to 375°F (190°C). Bring a large pot of salted water to a boil and cook the pasta until just shy of al dente. Drain and set aside.

In a large saucepan over medium heat, melt the butter. When it foams and begins to bubble, sprinkle in the flour and whisk constantly for about 1 minute to make a roux; it should be pale and slightly golden.

Slowly whisk in the milk (and chicken stock if using), stirring constantly until the mixture is smooth. Bring to a gentle simmer; the sauce will thicken and coat the back of a spoon in 3–5 minutes.

Lower the heat and add the Dijon, smoked paprika, salt, and pepper. Stir in the cheddar and mozzarella a handful at a time until silky and fully melted.

Fold the cooked pasta and shredded chicken into the cheese sauce, stirring until evenly coated. If the mixture feels too thick, add a splash of milk to loosen it.

Pour half of the mac and cheese into a buttered baking dish, dot with a few extra bits of cheese, then add the rest. Sprinkle the panko-butter mixture evenly across the top and finish with a light dusting of smoked paprika.

Bake for 15–20 minutes until the top is golden and the cheese is bubbling at the edges. For extra brownness, place under the broiler for 1–2 minutes, watching closely to prevent burning.

Let the dish rest for 5 minutes before serving.