Cook the macaroni according to the packet instructions, but take off 2 minutes of cooking time. Drain in a colander and rinse under the cold tap to stop it cooking, then drizzle some vegetable oil over it and toss the pasta, to stop it sticking together. Put it to one side while you make the sauce.
Carefully cut the top off the brie, just underneath the rind. Reserve the lid, then use a spoon to scoop out as much of the inside as possible - leaving 1 centimetre or so around the edges. Reserve the middle then put the brie case to one side for later.
Melt 1 tablespoon of butter in a small pan, add the flour and stir it constantly for 2 minutes, so the flour cooks. Gradually add in milk, using a whisk to ensure there are no lumps, until you have a smooth sauce. Cook for 5 minutes, stirring constantly, until it starts to thicken. Add 200 grams of the reserved brie in chunks, stirring it well until it melts, then take it off the heat, add the cooked pasta and mix it together well, making sure the macaroni is well covered in the sauce.
Pre-heat the oven to 220 C (200 C fan). Roll out 2 sheets of puff pastry to 0.5cm thick, and cut them 5cm wider than the brie wheel.
Put one circle of pastry onto a baking sheet, and place the brie case in the middle of that. Fill the case with the macaroni cheese, then place the brie lid on top. Put the second pastry circle on top, and use egg wash to stick the edges together. Brush the top of the pastry with egg wash then bake for 15 minutes until well risen and golden brown. Turn down the oven to 150 C (130 C fan) and cook for another 10 minutes.
Rest on the tray for 5 minutes, then transfer to a serving plate or board, and serve.