  Massive stars form deeply embedded in dense molecular gas, which they stir
and heat up and ionize. During an early phase, the ionization is confined to
hypercompact HII regions, and the stellar radiation is entirely absorbed by
dust, giving rise to a hot molecular core. To investigate the innermost
structure of such high-mass star-forming regions, we observed vibrationally
excited HCN (via the direct $\ell$-type transition of v2=1, $\Delta J$=0, J=13,
which lies 1400 K above ground) toward the massive hot molecular cores
G10.47+0.03, SgrB2-N, and SgrB2-M with the Very Large Array (VLA) at 7 mm,
reaching a resolution of about 1000 AU (0.1"). We detect the line both in
emission and in absorption against HII regions. The latter allows to derive
lower limits on the column densities of hot HCN, which are several times
$10^{19}$ cm$^{-2}$. We see indication of expansion motions in G10.47+0.03 and
detect velocity components in SgrB2-M at 50, 60, and 70 km/s relative to the
Local Standard of Rest. The emission originates in regions of less than 0.1 pc
diameter around the hypercompact HII regions G10.47+0.03 B1 and SgrB2-N K2, and
reaches brightness temperatures of more than 200 K. Using the three-dimensional
radiative transfer code RADMC-3D, we model the sources as dense dust cores
heated by stars in the HII regions, and derive masses of hot (>300 K) molecular
gas of more than 100 solar masses (for an HCN fractional abundance of
10$^{-5}$), challenging current simulations of massive star formation. Heating
only by the stars in the HII regions is sufficient to produce such large
quantities of hot molecular gas, provided that dust is optically thick to its
own radiation, leading to high temperatures through diffusion of radiation.
