  Motivated by the recent measurement of the upper limit of ${B}^0 \to J/\psi
D$ branching ratio, which is important in accounting for the soft $J/\psi$
production in B decays, we investigate ${B}^0 \to J/\psi D^{(\star)}$ and
$\eta_c D^{(\star)}$ decays in perturbative QCD approach based on $k_T$
factorization. Being pure annihilation (W-exchange) decays, these branching
ratios are estimated to be at the order of $10^{-5} \sim 10^{-7}$, which are
just at the corner of being observable at the $B$ factories. The measurements
of these decay channels may help us to understand the QCD dynamics in the
corresponding energy scale, especially the reliability of pQCD approach to
these processes.
