We present a design for an on-chip MEMS-actuated Variable Optical Attenuator (VOA) based on Silicon Photonic MEMS technology. The VOA consists of 30 individual mechanically movable MEMS cantilevers, suspended over an integrated, 1 mu m wide bus waveguide, each terminating with two optical attenuation bars. By exploiting the pull-in instability, electrostatic actuation allows to move the individual cantilevers into proximity of the waveguide, leading to scattering of the evanescent field and thus attenuation of the remaining optical power in the waveguide. Electrodes are placed below the cantilevers for electrostatic actuation. Mechanical stoppers are used to avoid contact between the cantilevers and the electrodes and to keep the bars at a precisely defined distance of 60 nm away from the bus waveguide. The attenuator provides nearly zero insertion loss in OFF state, while in ON state, the attenuation range is defined by the number of actuated digital attenuation cantilevers and can be adjusted in discrete increments of only 1.2 dB. Owing to the small size, fast microsecond scale response time can be achieved, and electrostatic MEMS actuation allows for broadband and low-power operation. Our design exhibits a compact footprint of 30 mu m x 45 mu m, attenuation from 0 dB to 36 dB, while keeping return loss below 27 dB. To the best of our knowledge, this is the first presentation of a design of a VOA in Silicon Photonic MEMS technology.