Life-long learning is necessary for all professions because the technologies, tools and skills required for success over the course of a career expand and change. Professionals in science, technology, engineering and mathematics (STEM) fields face particular challenges as new multi-disciplinary methods, e.g. Machine Learning and Artificial Intelligence, mature to replace those learned in undergraduate or graduate programs. Traditionally, industry, professional societies and university programs have provided professional development. While these provide opportunities to develop deeper understanding in STEM specialties and stay current with new techniques, the constraints on formal classes and workshops preclude the possibility of Just-In-Time Mastery Learning, particularly for new domains. The MIT Lincoln Laboratory Supercomputing Center (LLSC) and MIT Supercloud teams have developed online course offerings specifically designed to provide a way for adult learners to build their own educational path based on their immediate needs, problems and schedules. To satisfy adult learners, the courses are formulated as a series of challenges and strategies. Using this perspective, the courses incorporate targeted theory supported by hands-on practice. The focus of this paper is the design of Mastery, Just-in-Time MOOC courses that address the full space of hands-on learning requirements, from digital to analog. The discussion centers on the design of project-based exercises for professional technical education courses. The case studies highlight examples from courses that incorporate practice ranging from the construction of a small radar used for real world data collection and processing to the development of high performance computing applications.