Fill a large pot with lightly salted water and bring to a boil over high heat. Stir in the macaroni and cook uncovered, stirring occasionally, until the pasta has cooked through but is still firm to the bite, about 8 minutes. Drain well.
Combine 65g butter, flour, milk, salt and cheddar cheese in a large saucepan over medium heat; cook and stir until the cheese is melted and the mixture thickens, 7 to 10 minutes. Add the macaroni and stir to coat. Pour the mixture into a 20x30cm baking dish. Mix the 65g melted butter and crushed crackers together in a bowl; scatter the cracker mixture evenly over the macaroni mixture.
Bake in preheated oven until golden brown on top, about 45 minutes.