  We analyze the cold dark matter density profiles of 54 galaxy halos simulated
with FIRE-2 galaxy formation physics, each resolved within $0.5\%$ of the halo
virial radius. These halos contain galaxies with masses that range from
ultra-faint dwarfs ($M_\star \simeq 10^{4.5} M_{\odot}$) to the largest spirals
($M_\star \simeq 10^{11} M_{\odot}$) and have density profiles that are both
cored and cuspy. We characterize our results using a new analytic density
profile that extends the standard Einasto form to allow for a pronounced
constant-density core in the resolved innermost radius. With one additional
core-radius parameter, $r_{c}$, this "core-Einasto" profile is able to
characterize the shape and normalization of our feedback-impacted dark matter
halos. In order to enable comparisons with observations, we provide fitting
functions for $r_{c}$ and other profile parameters as a function of both
$M_\star$ and $M_{\star}/M_{\rm halo}$. In agreement with similar studies done
in the literature, we find that dark matter core formation is most efficient at
the characteristic stellar-mass to halo-mass ratio $M_\star/M_{\rm halo} \simeq
5 \times 10^{-3}$, or $M_{\star} \sim 10^9 \, M_{\odot}$, with cores that are
roughly the size of the galaxy half-light radius, $r_{c} \simeq 1-5$ kpc.
Furthermore, we find no evidence for core formation at radii $\gtrsim 100\ \rm
pc$ in galaxies with $M_{\star}/M_{\rm halo} < 5\times 10^{-4}$ or $M_\star
\lesssim 10^6 \, M_{\odot}$. For Milky Way-size galaxies, baryonic contraction
often makes halos significantly more concentrated and dense at the stellar
half-light radius than dark matter only runs. However, even at the Milky Way
scale, FIRE-2 galaxy formation still produces small dark matter cores of
$\simeq 0.5-2$ kpc in size. Recent evidence for a ${\sim} 2$ kpc core in the
Milky Way's dark matter halo is consistent with this expectation.
