Preheat the oven to 175 degrees Celsius.
If using flax egg combine ground flax and water in a small bowl. Set aside for a few minutes.
Combine milk and apple cider vinegar in a glass. Set aside.
Line a baking pan with parchment paper.
Combine the dry ingredients in a bowl.
Combine sweet potato mash, egg/flax egg, apple sauce, fiber syrup/agave/honey, stevia vanilla drops, and the milk mixture in a bowl. Whisk until combined.
Stir in the dry ingredients and mix just until combined.
Pour into the lined baking pan and bake in the oven for 30-40 minutes, or until a toothpick comes out clean.
Remove from the oven and let it cool completely before slicing it.