Bring a large pot of salted water to a boil and cook pasta according to package directions. Drain well, reserving about ½ cup of the pasta water.

In a blender or food processor, combine yogurt, ricotta, parmesan, butter, and lemon zest. Blend for 1 to 2 minutes or until completely smooth.  Stream in reserved pasta water if desired to achieve desired taste and consistency.

Toss the cheese and yogurt mixture with the cooked pasta, return to the pot and cook over low heat for 5 minutes or until just warmed. Season with salt and pepper to taste. Divide between bowls and serve warm.