The Spartans' (10-1, 7-0 Big Ten) clinched the Legends Division title and advanced to the Big Ten title game with their seventh consecutive victory.
Cook completed 16-of-23 passes for 293 yards. Langford carried 25 times for 150 yards and caught two passes for 35 yards.
Northwestern (4-7, 0-7) led 3-0 at the end of the first quarter, but lost starting quarterback Kain Colter to an arm injury on a jarring hit by safety Isaiah Lewis, who was ejected on a targeting penalty.
Trevor Seiman took over and completed 25-of-43 passes for 227 yards, but was intercepted twice and was unable to get the ball into the end zone.
The Wildcats, who lost their seventh game in a row, got two field goals from Jeff Budzien.