I understand that diffraction patterns are seen with single slits with multiple photons/electrons, due to Fraunhofer effects. But do multiple single photons/electrons produce the same pattern, as they would in the double-slit experiment? (i.e. single photon detections aggregated over time)
The diffraction pattern seen when you have two "real" slits is actually not a pure cosine function; since the diffraction pattern is the Fourier Transform of the aperture function, and since the aperture function is the convolution of a single ("wide") slit with two "infinitely narrow" slits, the actual pattern observed is the product of a single slit pattern (sinc function) with the cosine function of the "ideal" double slit.
In other words - the "double slit" experiment observed both the "single slit" diffraction pattern, and the "double slit" diffraction pattern.
So the answer is "yes, a single slit diffraction experiment works, even when you use a single photon/electron at a time".
For reference, this site shows the result of a single-photon-double-slit experiment, which clearly shows the envelope (due to the finite width of the individual slits):