  We present preliminary results on the branching fractions $BF(B^0\to
D_s^{\ast+} D^{\ast-})$ and $BF(D_s^+\to\phi\pi^+)$, based on a data sample of
approximately $124\times10^6$ \BB events collected by the \babar detector at
the PEP-II $e^+ e^-$ \B--factory. $BF(B^0\to D_s^{\ast+} D^{\ast-})$ is
measured selecting neutral \B meson decays to the final state $D_s^{\ast+}
D^{\ast-}$ with partial reconstruction of the $D_s^{\ast+}$, in which only the
$D^{\ast-}$ and the soft photons from the decay $D_s^{\ast+} \to D_s^+ \gamma$
are reconstructed. The branching fraction product $BF(B^0\to D_s^{\ast+}
D^{\ast-}) \cdot BF(D_s^+\to\phi\pi^+)$ is measured via a complete
reconstruction of the whole decay chain. Comparing these two measurements
provides a model-independent determination of the $BF(D_s^+\to\phi\pi^+)$
branching fraction. We obtain the following preliminary results:
  $BF(B^0\to D_s^{\ast+} D^{\ast-}) = (1.85 \pm 0.09 \pm 0.16)%$,
  $BF(D_s^+\to\phi\pi^+) = (4.71 \pm 0.47 \pm 0.35)%$ where the first error in
each measurement is statistical, the second systematic.
