Bring 8 cups water to a boil in a medium pot. Add pasta. Boil 8 minutes or until al dente. Rinse under cool water and set aside.
Add butter to pot and place over medium heat. Add garlic. Saute garlic in butter 2 minutes, stirring frequently. Whisk in flour until smooth. Continue to cook, whisking frequently, 2 minutes. Slowly whisk in 4 cups milk. Whisk until smooth.
Combine remaining milk and cornstarch. Stir cornstarch slurry into pot, stirring until smooth. Bring to a boil and stir until thickened.
Stir pasta, cheeses, and kale into the sauce; keep stirring until cheese is melted and everything is heated through.