Cook macaroni in large pot with salt until just tender. Reserve 1/2 cup water; drain.
Meanwhile, cook bacon in large skillet over medium 8 to 10 minutes until crisp. Plate; leave exactly 2 tablespoons fat and discard excess.
Whisk flour into fat 1 minute; gradually whisk in milk. Add mustard and pepper; simmer 4 minutes until lightly thickened.
Remove from heat; stir cheddar smooth. Fold in pasta and half bacon, loosening with reserved water by tablespoon if needed.
Top with remaining bacon; serve promptly.