  We investigate the spatial, temporal, and spectral properties of 10
microflares from AR12721 on 2018 September 9 and 10 observed in X-rays using
the Nuclear Spectroscopic Telescope ARray (NuSTAR) and the Solar Dynamic
Observatory's Atmospheric Imaging Assembly and Helioseismic and Magnetic Imager
(SDO/AIA and HMI). We find GOES sub-A class equivalent microflare energies of
10$^{26}$-10$^{28}$ erg reaching temperatures up to 10 MK with consistent
quiescent or hot active region core plasma temperatures of 3-4 MK. One
microflare (SOL2018-09-09T10:33), with an equivalent GOES class of A0.1, has
non-thermal HXR emission during its impulsive phase (of non-thermal power
$\sim$7$\times$10$^{24}$ erg s$^{-1}$) making it one of the faintest X-ray
microflares to have direct evidence for accelerated electrons. In 4 of the 10
microflares, we find that the X-ray time profile matches fainter and more
transient sources in the EUV, highlighting the need for observations sensitive
to only the hottest material that reaches temperatures higher than those of the
active region core ($>$5 MK). Evidence for corresponding photospheric magnetic
flux cancellation/emergence present at the footpoints of 8 microflares is also
observed.
