Fill a large bowl with warm water; add udon and let sit for 3 minutes. Drain and set aside.
Heat oil in large skillet over medium heat. Add bacon and cook, stirring, for 2 minutes. Add onion and cook for 3 minutes; add kimchi and cook until kimchi is lightly caramelized, 4 to 5 minutes.
Stir in ¼ cup of the water, kimchi juice and gochujang and bring to a boil; stir in noodles.
Meanwhile, in bowl, stir together egg yolks, remaining ¼ cup water, ¼ cup of the cooking liquid with some kimchi and Parmesan. Scrape into noodle mixture with butter and salt, stirring to coat. Cook, tossing, until just hot; remove from heat.
Divide udon mixture among plates and sprinkle with nori and sesame seeds.