In a large pot, cook the shells as directed on the package. Reserve 1/2 cup of the pasta cooking liquid. Then, rinse with hot water and drain. Return the shells to the pot.
In a large skillet, heat half of the oil over medium heat. Add the garlic and saute until fragrant, about 30 seconds. Add the spinach and basil and cook, covered, for 2 minutes.
Uncover and cook, stirring, until no liquid remains, about 2 more minutes. Transfer the spinach-basil mixture to a food processor. Add the remaining oil, kosher salt, garlic powder and freshly ground pepper. Puree until smooth. Add the reserved pasta cooking liquid and continue blending.