Cook pasta according to package instructions. Reserve 1 cup cooking water, then drain.
Meanwhile, in a large pot over medium heat add olive oil then add garlic and cook for 3 minutes stirring occasionally until golden in color. Add the clams, red pepper flakes and ½ teaspoon salt; cover, increase the heat to high and cook until the clams open, about 7 minutes. Discard any unopened clams.
Add the butter, lemon zest and pasta to the pot with the clams; toss to coat, about 1 minute. Add some of the reserved pasta cooking water to loosen the sauce if the pasta seems dry.
Stir in the parsley and season with salt and pepper. Serve drizzled with olive oil.