You can't get more direct than this in terms of proving that photons exist!
The particular case of the double-slit experiment will be discussed at: single particle double slit experiment.
Bibliography:
- https://iopscience.iop.org/book/978-0-7503-3063-3.pdf Quantum Mechanics in the Single Photon Laboratory by Waseem, Ilahi and Anwar (2020)