During the years that she was addicted to online gaming, life for Emily Kim began when she got home from work at 6 p.m. “I would shower quick, and eat something, no matter what, so I could start playing my game,” said Ms. Kim, a.k.a the YouTube Korean-cooking star Maangchi. “And I wouldn’t stop till 3 a.m.” In 2003, divorced and with her two grown children out of the house, Ms. Kim ventured into the online role-playing battle game City of Heroes and couldn’t pull herself away. Maangchi, pronounced MAHNG-chee and meaning “hammer” in Korean, was the name of her...