Massive open online courses (MOOCs) have grown in popularity since 2008 but suffer from low completion rates, often as low as 2%. This study examines how designing MOOCs with consideration of cognitive load can improve retention and learning outcomes, particularly for adult learners such as educators. Employing a design-based research methodology, five learning modules were developed and tested with pre-service teachers. The modules incorporated features like avatars, scenarios, feedback, multimedia, and accessibility considerations. Cognitive load was measured using an adapted NASA Task Load Index. Out of 247 participants, 175 completed surveys. Overall, participants reported low cognitive load. Qualitative themes included the helpfulness of scenario features, disconnect between simulations and the real world, and a desire for more strategies and examples. The primary concern was the transferability of learning to real classrooms. Future iterations should increase exposure to diverse scenarios to build adaptive expertise. With cognitive load-informed design, MOOCs can provide effective learning opportunities for busy adult professionals.