Mathematics teacher education programs in the United States are charged with preparing prospective secondary teachers (PSTs) to teach reasoning and proving across grade levels and mathematical topics. Although most programs require a course on proof, PSTs often perceive it as disconnected from their future classroom practice. Our design research project developed a capstone course "Mathematical Reasoning and Proving for Secondary Teachers" and systematically studied its effect on PSTs' content and pedagogical knowledge specific to proof. This paper focuses on one course module--"Quantification and the Role of Examples in Proving," a topic which poses persistent difficulties to students and teachers alike. The analysis suggests that after the course, PSTs' content and pedagogical knowledge of the role of examples in proving increased. We provide evidence from multiple data sources: pre-and post-questionnaires, PSTs' responses to the in-class activities, their lesson plans, reflections on lesson enactment, and self-report. We discuss design principles that supported PSTs' learning and their applicability beyond the study context.