This paper explores possible affordances of technology and online professional learning to develop and support communities of practice in which educational practitioners develop, share and build on one another's reflections and learning. Evidence is presented from a design-based research study that iteratively designed and developed a mechanical MOOC on educational dialogue for local facilitators. Local facilitators are practitioners who take on a leadership role whereby they coordinate and support peers in their setting in developing their dialogic practices. The research employs mixed methods for collection and analysis of data from course participants, exploring the successes and challenges of online professional development courses within 10 thematic areas. Design principles are offered for each of these 10 themes for scalable and sustainable online professional learning programmes that promote practitioner reflection, agency and empowerment, and view educators as valuable creators and contributors to professional learning resources and environments.