  Borexino is an experiment for low energy neutrino spectroscopy at the Gran
Sasso underground laboratories. It is designed to measure the mono-energetic
$^7$Be solar neutrino flux in real time, via neutrino-electron elastic
scattering in ultra-pure organic liquid scintillator. Borexino has the
potential to also detect neutrinos from the \emph{pep} fusion process and the
CNO cycle. For this measurement to be possible, radioactive contamination in
the detector must be kept extremely low. Once sufficiently clean conditions are
met, the main background source is $^{11}$C, produced in reactions induced by
the residual cosmic muon flux on $^{12}$C. In the process, a free neutron is
almost always produced. $^{11}$C can be tagged on an event by event basis by
looking at the three-fold coincidence with the parent muon track and the
subsequent neutron capture on protons. This coincidence method has been
implemented on the Borexino Counting Test Facility data. We report on the first
event by event identification of \emph{in situ} muon induced $^{11}$C in a
large underground scintillator detector. We measure a $^{11}$C production rate
of 0.130 $\pm$ 0.026 (stat) $\pm$ 0.014 (syst) day$^{-1}$ ton$^{-1}$, in
agreement with predictions from both experimental studies performed with a muon
beam on a scintillator target and \emph{ab initio} estimations based on the
$^{11}$C producing nuclear reactions.
