Prep Potato: Roast or steam the sweet potato until very soft. Scoop the flesh into a mixing bowl and mash until no lumps remain.
Combine: Add the ripe banana to the bowl and mash it into the potato. Whisk in the 2 eggs and the cocoa powder.
Smooth: Continue mashing and stirring with a fork until a thick, uniform batter forms.
Fold: Stir in most of the dark chocolate chips, saving a few to sprinkle on top.
Bake: Pour the batter into a square baking dish lined with parchment paper. Spread it evenly with a spatula. Sprinkle the remaining chips on top.
Cook: Bake at 180°C (350°F) for 20–25 minutes. The edges should be set, but the center will still feel quite soft.
The "Set" Step: This is crucial—let the brownies cool completely in the pan before slicing. Because they are flourless, they need that cooling time to firm up into a fudgy consistency.