  We analyze the binary microlensing event MOA-2016-BLG-231, which was observed
from the ground and from Spitzer. The lens is composed of very low-mass brown
dwarfs (BDs) with $M_1 = 21^{+12}_{-5} \ M_J$ and $M_2 = 9^{+5}_{-2}\ M_J$, and
it is located in the Galactic disk $D_{\rm L} = 2.85^{+0.88}_{-0.50}\ {\rm
kpc}$. This is the fifth binary brown dwarf discovered by microlensing, and the
BD binary is moving counter to the orbital motion of disk stars. Constraints on
the lens physical properties come from late time, non-caustic-crossing features
of the Spitzer light curve. Thus, MOA-2016-BLG-231 shows how Spitzer plays a
crucial role in resolving the nature of BDs in binary BD events with short
timescale ($\lesssim 10$ days).
