Place a large pot of generously salted water over high heat, and bring to a boil. Add spaghetti and cook until al dente according to package instructions.
Ladle out a cup of the hot pasta water and set aside. Then drain the pasta and return it to a dry pot with 1 tb of butter.
Add the kale pesto, a splash of the pasta water, and the grated parmesan cheese. Toss until all the pasta is coated, adding more pasta water as needed to loosen the sauce and make it creamy. Serve immediately.