Non-centrosymmetric stacking in boron nitride (BN) enables out-of-plane polarization, offering a route toward 2D dielectricswith embedded nonvolatile functionality. However, achieving uniform, wafer-scale growth of metastable Bernal-stacked BN (bBN)remains challenging, as it requires breaking interlayer inversion symmetry. Here, we demonstrate the wafer-scale epitaxial growthof single-crystal bBN bilayers on Ni(111)/sapphire substrates using metal-organic chemical vapor deposition. By employing a flow-modulation epitaxy approach, we enhanced surface reaction kinetics while suppressing secondary nucleation, thereby enablingself-limiting growth of uniform bilayer bBN films. Monoatomic Ni step edges deterministically enforced the AB stacking order,energetically stabilizing the bilayer configuration by lifting the degeneracy of competing stacking orientations, as confirmed byatomic-resolution imaging and density functional theory calculations. The resulting bBN films served as atomically thin dielectricinterlayers in top-gated molybdenum disulfide (MoS₂) transistor arrays, yielding spatially uniform device performance across thewafer and enabling nonvolatile ferroelectric switching of the MoS₂ channels via polarization reversal in the bBN interlayer. Theseresults expand the understanding of step-edge-guided growth mechanisms for controlling stacking order and layer number inmetastable 2D materials, and demonstrate a reliable wafer-scale approach to phase-controlled synthesis of single-crystal bBNbilayers with their integration as functional 2D dielectrics for practical electronic applications.