We propose and experimentally demonstrate topgated complementary n- and p-type black phosphorous FETs by engineering the workfunction of pre-patterned electrodes embedded in a SiO2 layer. Pre-patterned electrodes offer the possibility of reducing the exposure time of exfoliated flakes to oxidant agents with respect to top-contacted devices and maximize the accessible area for sensing applications. The devices are realized by exfoliating multilayer black phosphorous flakes on top of pre-patterned embedded source and drain contacts. A capping layer consisting of 15 nm thick Al2O3 is used to prevent flakes degradation and serves as top gate dielectric. We deposited both Au and Ag contacts to investigate the impact of the electrode workfunctions on BP FETs polarity. Au contacted devices showed p-type conduction with ON/OFF current ratio 140 and holes mobility up to 40 cm2V-1s-1. Devices with Ag contacts showed prevalent n-type conduction with ON/OFF ratio 1700 and electron mobility 2 cm2 V-1s-1. The reported results represent a substantial improvement with respect to reported alternative implementations of black phosphorous FETs with pre-patterned, non-embedded electrodes. Moreover, we demonstrate that Ag is a promising metal for electron injection in black phosphorous FETs.