Preheat oven to 350 degrees F. Line an 8 x 8-inch baking pan with foil and lightly spray with nonstick cooking spray.
In a medium bowl, beat together Japanese sweet potato, butter, sugar, apple cider vinegar, vanilla extract and milk.
Add flour, baking powder, baking soda and salt, mixing until combined. Fold in chocolate chunks.
Spread evenly in prepared pan and bake 27-32 minutes or until a toothpick inserted near the center comes out clean or with a few moist crumbs. Let cool completely before cutting and serving.