  We present high resolution ($\sim0.14$" = 710 pc) ALMA [CII] 158$\mu$m and
dust continuum follow-up observations of REBELS-25, a [CII]-luminous
($L_{\mathrm{[CII]}}=(1.7\pm0.2)\times 10^9 \mathrm{L_{\odot}}$) galaxy at
redshift $z=7.3065\pm0.0001$. These high resolution, high signal-to-noise
observations allow us to study the sub-kpc morphology and kinematics of this
massive ($M_* = 8^{+4}_{-2} \times 10^9 \mathrm{M_{\odot}}$) star-forming
(SFR$_{\mathrm{UV+IR}} = 199^{+101}_{-63} \mathrm{M_{\odot}} \mathrm{yr}^{-1}$)
galaxy in the Epoch of Reionisation. By modelling the kinematics with
$^{\mathrm{3D}}$BAROLO, we find it has a low velocity dispersion ($\bar{\sigma}
= 33 \pm 9$ km s$^{-1}$) and a high ratio of ordered-to-random motion
($V_{\mathrm{rot, ~max}}/\bar{\sigma} = 11 ^{+8}_{-4}$), indicating that
REBELS-25 is a dynamically cold disc. Additionally, we find that the [CII]
distribution is well fit by a near-exponential disc model, with a S\'ersic
index, $n$, of $1.3 \pm 0.2$, and we see tentative evidence of more complex
non-axisymmetric structures suggestive of a bar in the [CII] and dust continuum
emission. By comparing to other high spatial resolution cold gas kinematic
studies, we find that dynamically cold discs seem to be more common in the high
redshift Universe than expected based on prevailing galaxy formation theories,
which typically predict more turbulent and dispersion-dominated galaxies in the
early Universe as an outcome of merger activity, gas accretion and more intense
feedback. This higher degree of rotational support seems instead to be
consistent with recent cosmological simulations that have highlighted the
contrast between cold and warm ionised gas tracers, particularly for massive
galaxies. We therefore show that dynamically settled disc galaxies can form as
early as 700 Myr after the Big Bang.
