Preheat oven to 325 degrees. Grease an 8-inch baking pan with coconut or olive oil, set aside.
In a large bowl, beat together the avocado and sweet potato until smooth (I used a hand mixer).
Add the maple syrup, milk, vanilla, beating until smooth and well combined.
Stir in the flour, cocoa, tapioca/arrowroot flour, flax meal, baking soda, and salt until well incorporated.
Stir in the walnuts and chocolate chips (if using).
Pour batter into prepared pan, smooth the top, and bake for 30 to 35 minutes.
Remove from oven and place on a wire cooling rack. Allow to cool completely before serving.
Notes
I have seen some Paleo sites use these chocolate chips, so I decided to add them. They are gluten-free, dairy-free, and soy-free, but do contain evaporated can juice.