Whisk together flour, cocoa, vanilla, cream of tartar, potassium bicarbonate, and salt.
Place sweet potatoes, nut butter, and syrup in large mixing bowl and stir until well combined.
Add dry ingredients to wet mixture and stir into thick batter.
Stir in chocolate chips and walnuts.
Transfer batter to parchment-lined 9 x 9 baking pan and smooth into even layer.
Bake for 40 minutes.
Remove parchment from pan, place on cooling rack, and cool completely before cutting into squares.
Notes
Any nut or seed butter should work in place of peanut butter.Batter is extremely thick, like heavy cookie dough. It takes a few minutes to press it into an even layer throughout the pan and level it out.