Preheat oven to 350
Boil pasta, cook until just done, do not overcook your pasta!
Coat a 9x12 baking dish with cooking spray
Heat the butter in a heavy bottomed saucepan. whisk in flour to combine.
Slowly whisk in the milk to prevent lumps from forming; then bring the mixture to a boil and cook until it thickens slightly about 5 minutes.
Add cheddar cheese salt and puree with an immersion blender.
Whisk the cheese mixture until smooth.
Stir the cheese mixture into the pasta gently with a large spoon .
Transfer the pasta and cheese mixture to baking dish
Sprinkle the top with parmesan.
Bake until bubbly, about 15-20 mintes