In a large bowl, combine 2 cups mashed potatoes, 2 large egg yolks, 1 cup fl our, and 1/2 cup shredded Parmesan (optional). Using your hands or a bench scraper, knead ingredients until a cohesive dough forms. Turn out onto a heavily floured surface and divide into 4 equal pieces. Roll pieces into long ropes about 3/4" in diameter and cut lengthwise into 1" pieces. Cook gnocchi in boiling water until they fl oat, 2 to 3 minutes. Drain, toss with your favorite sauce, and serve.