Preheat the oven: Preheat the oven to 350°F (175°C). Grease or line a 9 x 9 inch (or similar) baking pan with parchment paper.
Mix the wet ingredients: In a large bowl, whisk the mashed sweet potato, melted butter, and brown sugar until smooth. Then add the eggs and vanilla extract, mixing well until well combined.
Add the dry ingredients: Sift the cocoa powder, flour, baking soda, and salt. Mix gently until combined.
Add the chocolate chips: Fold in the chocolate chips, reserving some for sprinkling if desired.
Bake: Pour the batter into the prepared pan and spread evenly. Optionally, sprinkle the top with more chocolate chips. Bake for 25 minutes, or until a toothpick inserted near the center comes out with some moist crumbs (but not wet batter).
Cool and serve: Let cool for about 10 minutes before cutting into squares.