Preheat oven to 350°F.
Lightly coat an 8” x 8” brownie/cake pan or a 9” round cake pan with coconut or other oil, and fit the bottom of the pan with a small piece of parchment paper.
In a blender (or using a handheld blender and a deep cup or vessel), puree the sweet potato, 1/2 cup of the water, maple syrup, balsamic vinegar, and vanilla extract until completely smooth. In a large bowl, combine the flour, coconut sugar, chocolate chips, and sea salt, then sift in the cocoa, baking powder, and baking soda.
Add the wet ingredients to the dry (be sure to scrape out all the blended ingredients with a spatula, and use the remaining 1 tablespoon of water to rinse the blender jar and get out any remaining puree). Mix until just well incorporated. Transfer to the prepared pan, bake for 21–23 minutes, remove, and let cool on a cooling rack.
To prepare the frosting: Place the sweet potato, coconut sugar, cocoa powder, cashew butter, sea salt, 1–2 tablespoons of the milk, and vanilla extract in a blender or food processor and puree until very smooth.
Taste, and add more sweetener if desired, and also another 2–3 tablespoons of milk if needed to thin to preferred consistency. Puree until smooth, scraping down the blender/processor bowl as needed.
Puree until as smooth as possible. Transfer to a container and refrigerate until ready to use.
Assemble cake, using desired toppings, serve and enjoy!