  We present high resolution ($\sim$300 au) Atacama Large
Millimeter/submillimeter Array (ALMA) observations of the massive young stellar
object G11.92-0.61 MM 1. We resolve the immediate circumstellar environment of
MM 1 in 1.3 mm continuum emission and CH$_{3}$CN emission for the first time.
The object divides into two main sources - MM 1a, which is the source of a
bipolar molecular outflow, and MM 1b, located 0.57'' (1920 au) to the
South-East. The main component of MM 1a is an elongated continuum structure,
perpendicular to the bipolar outflow, with a size of $0.141'' \times 0.050''$
($480\times170$ au). The gas kinematics toward MM 1a probed via CH$_{3}$CN
trace a variety of scales. The lower energy $J=12-11$ $K=3$ line traces
extended, rotating gas within the outflow cavity, while the $v$8=1 line shows a
clearly-resolved Keplerian rotation signature. Analysis of the gas kinematics
and dust emission shows that the total enclosed mass in MM 1a is $40\pm5$
M$_{\odot}$ (where between 2.2-5.8 M$_{\odot}$ is attributed to the disk),
while MM 1b is $<0.6$ M$_{\odot}$. The extreme mass ratio and orbital
properties of MM 1a and MM 1b suggest that MM 1b is one of the first observed
examples of the formation of a binary star via disk fragmentation around a
massive young (proto)star.
