In classrooms today, mathematics teachers need to have digital competencies to digitally enrich their teaching. However, research has shown that even prospective and beginning teachers are not able to efficiently integrate digital tools into their teaching. Therefore, university teacher training should focus on the acquisition of digital competencies. For this purpose, we developed a university training course aimed at fostering prospective mathematics teachers’ digital competencies by enabling them to design and use digital learning environments using the digital tool STACK. The key elements of the training course and one prospective teacher’s experiences are discussed in this paper. The results indicate that this course concept has a positive impact on the development of digital competencies.