  The formation of the most massive O-type stars is poorly understood. We
present a case study of a young massive clump from the ATLASGAL survey,
G328.2551-0.5321. It exhibits a bolometric luminosity of 1.3$\times$10$^4$
L$_{\odot}$ corresponding to a current protostellar mass of $\sim$11 and 16
M$_{\odot}$. We analyze high angular-resolution observations with ALMA at
$\sim$0.17" corresponding a physical scale of $\sim$400 au in dust continuum
and molecular lines. The dust continuum emission reveals a single high-mass
protostellar envelope and shows evidence for a marginally resolved continuum
source. We detect a rotational line of CH$_3$OH within its $v_{\rm t}$=1
torsionally excited state revealing two bright peaks of emission spatially
offset from the dust continuum peak, and exhibiting a distinct velocity
component $\pm$4.5 km s$^{-1}$ offset compared to the source $v_{\rm lsr}$.
Local thermodynamic equilibrium analysis suggests
N(CH$_3$OH)=1.2$-$2$\times$10$^{19}$ cm$^{-2}$, and kinetic temperatures of
160$-$200 K at the position of these peaks. Their velocity shifts correspond
well to the expected Keplerian velocity around a central object with
15M$_{\odot}$ consistent with the mass estimate based on the source's
bolometric luminosity. We propose a picture where the CH$_3$OH emission peaks
trace the accretion shocks around the centrifugal barrier, pinpointing the
interaction region between the collapsing envelope and an accretion disk.
Because the HC$_3$N $v_{\rm 7}$=1e ($J$=38-37) line shows compact emission, and
a velocity pattern consistent with models of Keplerian rotation, we suggest
that this could be a new tracer for compact accretion disks around high-mass
protostars. The estimated physical properties of the accretion disk suggest a
specific angular momentum several times larger than typically observed towards
low-mass protostars.
