Preheat the oven. Set your oven to 170°C (300°F).
In a frying pan over medium heat, melt 60g of the butter. Add the garlic and cook for 1-2 minutes until fragrant. Add the prawns and coat them in the buttery garlic. Cook for 1-2 minutes until just pink. Don't cook them all the way through as they'll finish cooking in the oven. Set aside.
In a large saucepan, cook the macaroni in salted boiling water according to packet instructions until al dente. Drain and set aside.
In another pan over medium heat, melt 60g of butter. Add the flour and whisk vigorously for 2-3 minutes until it forms a paste.
Once the flour has cooked out, gradually whisk in the heated milk until well combined. Continue whisking for 5-6 minutes over medium heat until the roux thickens. Add the white pepper, Dijon mustard, and cheddar cheese, whisking until the cheese has melted completely.
Stir the cooked macaroni and prawns into the cheese sauce until everything is well coated.
Melt the remaining 50g of butter in the microwave (cover it to prevent splattering). In a bowl, combine the melted butter, panko breadcrumbs, and Parmesan cheese.
Transfer the mac and cheese to a baking dish. Sprinkle the breadcrumb mixture evenly over the top. Bake for 15-20 minutes until golden and crispy on top.
Remove from the oven and garnish with freshly chopped parsley. Serve immediately.