Preheat the oven to 375 degrees F and grease a standard size 12 cup nonstick muffin tin well. (I use classic Pam spray.)
Add the sweet potato, milk, butter, maple syrup, egg, and vanilla to a medium bowl. Stir together, going slowly and gently, until fully combined together.
Stir in the flours, baking soda, cinnamon, and salt.
To add chocolate chips, sprinkle a few on top of each muffin before baking. To add lemon zest (without chocolate), stir 1 tablespoon fresh lemon zest into the batter.
Divide among the prepared muffin tin, using about 1/4 cup batter in each, and bake for 16-20 minutes or until a cake tester inserted into the center of a muffin comes out cleanly. (In my oven it was right at 18 minutes. To make as mini muffins, bake for about 12-14 minutes.) Let cool for a minute in the pan, then transfer to a wire rack to cool.
Serve slightly warm or at room temperature.