A recently completed calculation of NNLO QCD corrections to the weak radiative $B$-meson decay is going to be reported. Contrary to previous estimates of these corrections, no interpolation in the charm quark mass is necessary. To achieve this goal, hundreds of four-loop propagator diagrams with unitarity cuts had to be calculated in the framework of the Low-Energy Effective Field Theory (LEFT) that is obtained from the SM via decoupling of the $W$ boson and heavier particles.