In my talk I will present a new method to measure the mass of accreting black hole systems through reverberation mapping. Both galactic and supermassive black holes display characteristic features in their energy spectra, including an Fe K$\alpha$ line and a Compton hump, that result from reprocessing of hard X-ray photons by the accretion disk. These features are distorted by relativistic effects and therefore they probe the geometry of the system. However, these distortions are insensitive to black hole mass, since they depend on disk geometry in units of gravitational radii. Measuring the reverberation lag resulting from the difference in path length between direct and reflected emission gives the length of the gravitational radius that can be constrained by a combined spectral-timing analysis, providing a means to measure black hole mass. We developed a model to jointly fit the time-averaged X-ray spectrum and the cross-spectrum as a function of energy for a range of Fourier frequencies, in order to constrain the mass of the Cygnus X-1 black hole. I will show how introducing a radial ionisation profile in the disk changes our results and I will compare our reverberation mass measurement with the dynamical mass measurement of Cygnus X-1.