Top left: Balmer break strength (from rest-frame [3620,3720] Å and [4000,4100] Å ranges) distribution for the standard MILES stellar library (blue), and after applying a Calzetti dust law (with A_V = 1; dashed purple), and the steep attenuation law from the Prospector galaxy-only model (solid red). The black vertical line and shaded region show the measured break strength and 1σ uncertainty of The Cliff. Top right: Effective temperatures of all MILES stars (blue) and the stars that, after applying the steep dust law, fall within 1σ of The Cliff (red). Bottom: Representative sample of MILES spectra with strong Balmer breaks, convolved to the PRISM resolution at z = 3.5 and normalised at [4000,4100] Å. Dotted and solid lines show the stellar spectra without dust and with steep dust attenuation, respectively. In comparison to the spectrum of The Cliff, K giants, main-sequence A stars and horizontal branch stars fall short at <3645 Å and >5000 Å. Only massive supergiants with steep dust attenuation can match the strength and shape of the Balmer break (albeit with a mismatch at > 5000 Å), and suggests that an extraordinary, top-heavy IMF would be required to explain the rest-frame optical emission of The Cliff.