Bring a large pot of lightly salted water to the boil. Add pasta and cook for 8 to 10 minutes or until al dente; drain. Preheat oven to 180 C / Gas 4.
In medium saucepan over medium heat, combine butter, milk, mushroom soup and processed cheese. Stir until cheese is melted and mixture is smooth. Stir in cooked pasta. Pour into 2 litre baking dish and bake 20 minutes or until top is golden brown. Let stand 10 minutes and serve.