  We present our calculation of the two-loop corrections of ${\cal
O}(\alpha_t^2)$ to the neutral Higgs boson masses of the CP-violating
Next-to-Minimal Supersymmetric extension of the Standard Model (NMSSM). The
calculation is performed in the Feynman diagrammatic approach in the gaugeless
limit at vanishing external momentum. We apply a mixed
$\overline{\mathrm{DR}}$-on-shell (OS) renormalization scheme for the NMSSM
input parameters. Furthermore, we exploit a $\overline{\mathrm{DR}}$ as well as
an OS renormalization in the top/stop sector. The corrections are implemented
in the Fortran code NMSSMCALC for the calculation of the Higgs spectrum both in
the CP-conserving and CP-violating NMSSM. The code also provides the Higgs
boson decays including the state-of-the-art higher-order corrections. The
corrections computed in this work improve the already available corrections in
NMSSMCALC which are the full one-loop corrections without any approximation and
the two-loop ${\cal O}(\alpha_t \alpha_s)$ corrections in the gaugeless limit
and at vanishing external momentum. Depending on the chosen parameter point, we
find that the ${\cal O}(\alpha_t \alpha_s + \alpha_t^2)$ corrections add about
4-7% to the one-loop mass of the SM-like Higgs boson for
$\overline{\mathrm{DR}}$ renormalization in the top/stop sector and they reduce
the mass by about 6-9% if OS renormalization is applied. For an estimate of the
theoretical uncertainty we vary the renormalization scale and change the
renormalization scheme and show that care has to be taken in the corresponding
interpretation.
