We demonstrate controlled synthesis of discrete two-dimensional (2D) PbSe nanoplatelets (NPLs), with measurable photoluminescence, via oriented attachment directed by quantum dot (QD) surface chemistry. Halide passivation is critical to the growth of these (100) face-dominated NPLs, as corroborated by density functional theory studies. PbCl2 moieties attached to the (111) and (110) of small nanocrystals form interparticle bridges, aligning the QDs and leading to attachment. We find that a 2D bridging network is energetically favored over a 3D network, driving the formation of NPLs. Although PbI2 does not support bridging, its presence destabilizes the large (100) faces of NPLs, providing means for tuning NPL thickness. Spectroscopic analysis confirms the predicted role of thickness-dependent quantum confinement on the NPL band gap.