Pre-college engineering education has gained traction in U.S. schools over the past twenty years. This growth is evident with engineering emerging as a crosscutting discipline in the Next Generation Science Standards. However, the scarcity of professional development (PD) for K-12 teachers who want to teach engineering and the few PD opportunities with the characteristics shown to improve teacher learning, i.e., contact time, long-term support, and follow-up, suggest a need for new and innovative PD offerings for K-12 teachers. We developed a 45-hour graduate course specifically for K-12 teachers to incorporate engineering into their classrooms and evaluated the effect of this long-term PD on K-12 teaching engineering self-efficacy. Additionally, this study looked at how the participants translated the course into their teaching practices. Forty-one in-service teachers participated either in a 2018 or 2019 semester-long course that combined nanotechnology content and Project-Based Learning pedagogy. Pre-post measures using Teaching Engineering Self-Efficacy Scale revealed significant gains in both cohort teachers' self-efficacy. Teachers found the course effective in building their technical skills and providing beneficial PD. This study's significant positive outcomes indicate that the course analyzed serves as a pre-college engineering education PD model.