Bring a big pot of salted water to the boil. Strip the kale from its stalks, removing only the big woody ones if the kale is young. Add the kale and garlic cloves to the boiling water and cook for about 10 minutes. Strain in a colander and squeeze out the excess water with tongs, as it will be hot. Place the kale and garlic in a blender with enough olive oil to make a smooth, runny sauce. Season with salt and pepper. Cook the rigatoni in a pot of salted water, then drain, saving half a cup of cooking water. Add the kale sauce and cooking water and toss gently until all the pasta is coated. Serve with parmesan.