  Moir\'e superlattices of two-dimensional (2D) materials with a small twist
angle are thought to be flexoelectric, though unambiguous confirmation of their
flexoelectricity is challenging due to artifacts associated with commonly used
piezoresponse force microscopy (PFM). For example, unexpectedly small phase
contrast, rather than the predicted 180o, between opposite flexoelectric
polarization was reported in twisted bilayer graphene (tBG). Here we developed
a methodology to extract intrinsic moir\'e flexoelectricity using twisted
double bilayer graphene (tDBG) as a model system, probed by lateral PFM. For
small twist angle samples, we found that a vectorial decomposition is essential
to recover the small intrinsic flexoelectric response at domain walls from a
large background signal. The obtained three-fold symmetry of commensurate
domains with both phase and amplitude at domain walls is fully consistent with
our theoretical calculations. Incommensurate domains in tDBG at relatively
large twist angles can also be observed by this technique. Our work provides a
general methodology for unraveling intrinsic flexoelectricity in van der Waals
moir\'e superlattices while providing insights into engineered symmetry
breaking in centrosymmetric materials.
