  The intrinsic planar Hall effect has attracted intensive interest inspired by
recent experiments. Existing theories of this effect require three dimensional
orbital motion, or strong spin-orbit coupling of certain forms, which do not
exist in van der Waals thin films. Here, we uncover a new origin of the planar
Hall effect - as an intrinsic property of layer coherent electrons - that
allows its presence even in bilayer and trilayer atomically thin limit. As
examples, we show that the effect can be triggered by strain and interlayer
sliding respectively in twisted bilayer graphene and trilayer transition metal
dichalcogenides, where the effect features rich tunability and even stronger
magnitude than those induced by topological nodal structures in bulk materials.
The layer mechanism also provides a new route towards quantized Hall response
upon a topological phase transition induced by in-plane magnetic field. These
results unveil the unexplored potential of quantum layertronics and moir\'e
flat band for planar Hall transport.
