Cacio e pepe is notorious for having the sauce clump or curdle, and not come out smooth and silky. It is so notorious for this clumping that people have come up with their own methods, including beginning the sauce with cold water, or, using butter or olive oil (*gasp*), to aid in the amalgamation of the sauce. This recipe doesn't use any of those fancy (or non-authentic) techniques. But here are a few reasons why cacio e pepe can clump: Using a domestic romano, instead of a pecorino romano Not letting the pasta finish cooking in the pan, and not allowing it to release enough starch Not using a bronze die extruded pasta (this is a high quality pasta that releases a lot of starch) Grating the pecorino thickly instead of finely