  Seeing pristine material from the donor star in a Type Ia supernova (SN Ia)
explosion can reveal the nature of the binary system. In this paper, we present
photometric and spectroscopic observations of SN 2020esm, one of the
best-studied SNe of the class of "super-Chandrasekhar" SNe Ia (SC SNe Ia), with
data obtained $-12$ to +360 days relative to peak brightness, obtained from a
variety of ground- and space-based telescopes. Initially misclassified as a
Type II supernova, SN 2020esm peaked at $M_{B} = -19.9$ mag, declined slowly
($\Delta m_{15}(B) = 0.92$ mag), and had particularly blue UV and optical
colors at early times. Photometrically and spectroscopically, SN 2020esm
evolved similarly to other SC SNe Ia, showing the usual low ejecta velocities,
weak intermediate mass elements (IMEs), and the enhanced fading at late times,
but its early spectra are unique. Our first few spectra (corresponding to a
phase of $\gtrsim$10~days before peak) reveal a nearly-pure carbon/oxygen
atmosphere during the first days after explosion. This composition can only be
produced by pristine material, relatively unaffected by nuclear burning. The
lack of H and He may further indicate that SN 2020esm is the outcome of the
merger of two carbon/oxygen white dwarfs (WDs). Modeling its bolometric light
curve, we find a $^{56}$Ni mass of $1.23^{+0.14}_{-0.14}$ M$_{\odot}$ and an
ejecta mass of $1.75^{+0.32}_{-0.20}$ M$_{\odot}$, in excess of the
Chandrasekhar mass. Finally, we discuss possible progenitor systems and
explosion mechanisms of SN 2020esm and, in general, the SC SNe Ia class.
