Cook pasta in large saucepan as directed on package for al dente pasta. Drain well and return to saucepan. Keep warm.
Meanwhile, mix milk and flour in medium saucepan with wire whisk. Bring to simmer on medium-low heat, whisking constantly. Add cauliflower florets; cook 12 to 15 minutes or until fork-tender, stirring occasionally.
With center part of cover removed to let steam escape, puree mixture in batches in blender on high speed until smooth. Return pureed mixture to saucepan. (Or puree mixture in saucepan with an immersion blender.) Stir in remaining ingredients; whisk until smooth.
Pour cheese mixture over hot cooked macaroni; mix well. Let stand 2 minutes before serving.