  Theoretical models predict that core-collapse supernovae (CCSNe) can be
efficient dust producers (0.1-1.0 Msun), potentially accounting for most of the
dust production in the early Universe. Observational evidence for this dust
production efficiency is however currently limited to only a few CCSN remnants
(e.g., SN1987A, Crab Nebula). In this paper, we revisit the dust mass produced
in Cassiopeia A (Cas A), a ~330-year old O-rich Galactic supernova remnant
(SNR) embedded in a dense interstellar foreground and background. We present
the first spatially resolved analysis of Cas A based on Spitzer and Herschel
infrared and submillimetre data at a common resolution of ~0.6 arcmin for this
5 arcmin diameter remnant following a careful removal of contaminating line
emission and synchrotron radiation. We fit the dust continuum from 17 to 500
micron with a four-component interstellar medium (ISM) and supernova (SN) dust
model. We find a concentration of cold dust in the unshocked ejecta of Cas A
and derive a mass of 0.3-0.5 Msun of silicate grains freshly produced in the
SNR, with a lower limit of >=0.1-0.2 Msun. For a mixture of 50% of
silicate-type grains and 50% of carbonaceous grains, we derive a total SN dust
mass between 0.4 Msun and 0.6 Msun. These dust mass estimates are higher than
from most previous studies of Cas A and support the scenario of supernova
dominated dust production at high redshifts. We furthermore derive an
interstellar extinction map for the field around Cas A which towards Cas A
gives average values of A_V=6-8 mag, up to a maximum of A_V=15 mag.
