To achieve the challenging target of $1\%$ precision on luminosity determination at the high-luminosity LHC (HL-LHC) with an instantaneous luminosity of up to $7.5 \times 10^{34} \ \mathrm{cm}^{-2} \mathrm{s}^{-1}$, the CMS experiment will employ multiple luminometers with orthogonal systematics. A key component of the proposed system is a stand-alone luminometer, the Fast Beam Condition Monitor (FBCM), which is fully independent from the central trigger and data acquisition services and able to operate at all times at $40$ MHz providing bunch-by-bunch luminosity measurement. FBCM is foreseen to be placed inside the cold volume of the Tracker as it utilizes silicon-pad sensors exploiting the zero-counting algorithm for luminosity measurement. FBCM will also provide timing information with a few ns precision enabling the measurement of beam induced background (BIB). We report on the optimisation of the design and the expected performance of FBCM.