For the cheese sauce: In a medium saucepan over medium heat, add 4 tablespoons of the butter. When foamy, sprinkle in the flour and a pinch of salt. Whisk to combine, then continue whisking until flour is cooked, 2 to 3 minutes. Whisk in the milk, pouring it in a slow, steady stream. Then whisk in the cream. Bring to a simmer. Set aside 3/4 cup of the mild Cheddar; whisk in the remaining mild Cheddar and the sharp Cheddar, smoked Cheddar and Brie. Continue to stir for 2 minutes or until all the cheese is melted, then turn the heat off. Stir in the remaining 1 tablespoon butter until melted and incorporated.
For the macaroni: Fill a large pot halfway with water. Season with salt until it tastes like the sea. Bring to a boil, then add the macaroni. Stir, then simmer until macaroni is al dente, about 8 minutes. Drain.
Meanwhile, preheat the oven to 350 degrees F.
Place cooked macaroni into a heat-proof serving dish. Ladle cheese sauce all over, then sprinkle the reserved mild Cheddar on top. Place in the oven just until all of the Cheddar is melted and everything is heated through, 3 to 5 minutes.