Mentoring promotes underserved students' STEM persistence but it is difficult to scale up. Virtual agents can amplify mentors' experiences to larger audiences, which is particularly important for mentors from under-represented backgrounds and for underserved students with less access to mentors. This paper introduces MentorStudio, an online platform that allows real-life mentors to self-record and publish video-based conversational virtual agents. MentorStudio's goals are to increase speed, scheduling flexibility, and autonomy in creating intelligent virtual mentors. MentorStudio platform components are introduced, along with initial feedback regarding usability and acceptance collected from 20 STEM mentors who recorded virtual mentors. Ove...