  Applying long wavelength periodic potentials on quantum materials has
recently been demonstrated to be a promising pathway for engineering novel
quantum phases of matter. Here, we utilize twisted bilayer boron nitride (BN)
as a moir\'e substrate for band structure engineering. Small-angle-twisted
bilayer BN is endowed with periodically arranged up and down polar domains,
which imprints a periodic electrostatic potential on a target two-dimensional
(2D) material placed on top. As a proof of concept, we use Bernal bilayer
graphene as the target material. The resulting modulation of the band structure
appears as superlattice resistance peaks, tunable by varying the twist angle,
and Hofstadter butterfly physics under a magnetic field. Additionally, we
demonstrate the tunability of the moir\'e potential by altering the dielectric
thickness underneath the twisted BN. Finally, we find that
near-60{\deg}-twisted bilayer BN provides a unique platform for studying the
moir\'e structural effect without the contribution from electrostatic moir\'e
potentials. Tunable moir\'e polar substrates may serve as versatile platforms
to engineer the electronic, optical, and mechanical properties of 2D materials
and van der Waals heterostructures.
