Bring medium pot with water to a boil, then add a pinch of salt and pasta. Cook as per package instructions or until al dente, about 8-10 minutes.
Meanwhile, in a high speed blender, add cottage cheese, milk, cheddar cheese, butter, mustard, cornstarch, garlic powder, salt and pepper. Process until smooth.
Pour mixture into another medium pot and cook on low-medium heat just until thickened into a sauce, stirring constantly. Takes about 5 minutes. Do not overcook.
Drain pasta and add to the pot with the cheese sauce and stir gently to coat.
Serve hot and preferably immediately while it's the most creamy. You can always add more milk to thin it out.
Notes
Store: Refrigerate in an airtight container for up to 2-3 days. I think it's best to reheat it in a microwave. You can add a bit of milk or water and reheat in a small pot on the stove as well but it might burn a bit and you have to stir constantly. But it's possible.
Freeze: Do not freeze this mac and cheese as pasta generally does not freeze well.
Cook low and slow: To avoid sauce from separating, cook on low to medium heat. Very important!