The egg yolks add creaminess to this bright and flavorful pasta dish.
Cook pasta in a large pot of boiling salted water, stirring occasionally, until very al dente (pasta will still be opaque and very firm in the center). Drain pasta, reserving 1 ½ cups pasta cooking liquid.
Meanwhile, finely grate egg yolks on the small holes of a box grater andset aside.
Heat six tablespoons of butter in a large skillet over medium-high. Add lemon zest and juice and red pepper flakes, swirling pan to incorporate. Add pasta and one cup pasta cooking liquid and cook, tossing often and adding more cooking liquid to help finish cooking pasta, until pasta is al dente and sauce is thickened and coats pasta, about five minutes. Taste and season with salt and pepper.
Add pecorino and remaining butter to pasta and toss until melted. Serve pasta topped with reserved grated egg yolks.