Boil 3 cups of water and cook macaroni until soft along with required salt. Drain it and keep aside.
Mix all purpose flour and mustard powder [If you dont have mustard powder, just crush some mustard seeds(big variety) you have and use it.] in a bowl. Heat a heavy bottomed pan with butter in low flame. When butter melts, add the mixed all purpose flour and whisk it.
Add milk gradually by whisking it and mix to form a smooth sauce in medium or low flame. When it's about to boil, add the cheese and mix well until the cheese melts in low – medium flame. Do not allow the sauce to boil.
When the sauce is ready, pour it over the cooked pasta and mix well. Serve hot and enjoy!!