Preheat oven to 350 degrees and prepare a metal brownie pan by lining with parchment paper.
In a high-speed blender (I use Blendtec and the twister jar), combine sweet potato, milk, tahini and vanilla, pureeing until smooth. Once smooth, add chocolate chips and pulse/puree through to break up to a crumbly consistency in the puree.
Transfer mixture to a bowl, scraping out everything.
Add dry ingredients (except those for topping) and mix through until well combined.
Transfer to lined baking pan, using a spatula to even out, and smoothing to have edges just slightly higher than centre.
In one of your measuring cups or a small bowl, combine the milk and tahini and stir/whisk until smooth.
Drizzle that mixture in a few places on top of the brownies.
Add chocolate chips and coarse salt, and sugar if using.
Bake for 35 minutes.
Remove, and let cool. Cut into squares when just cooled (or refrigerate before cutting into squares for easier slicing). Serve!