In a medium bowl, whisk together the oil, vanilla, egg, Greek yogurt, and milk until it comes together into a smooth liquid. Pour over the dry ingredients. Add the melted and cooled chocolate (melt according to package instructions if using chocolate chips). Using a wooden spoon or a rubber spatula, begin to mix the wet and dry ingredients together. When the batter just starts to come together, add in the toasted walnuts and the chocolate chips. Continue to mix until a you see just a light dusting of flour, being careful not to over stir.