Peel and dice the sweet potato. Place in a saucepan, cover with water, and bring to a boil. Cook for 15-20 minutes, or until very tender. Drain thoroughly and mash with a potato masher or fork until smooth. Measure out 1.5 cups of puree. Set aside to cool slightly.

Preheat your oven to 350°F (175°C). Line a 9x13 inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the brownies out later. Lightly grease the parchment paper.

In a medium bowl, whisk together the flour, cocoa powder, baking powder, and salt. Set aside.

In a large mixing bowl, combine the melted butter, granulated sugar, and light brown sugar. Whisk until well combined and smooth. Beat in the eggs one at a time, then stir in the vanilla extract and the cooled sweet potato puree until fully incorporated.

Gradually add the dry ingredient mixture to the wet ingredient mixture, mixing with a rubber spatula until just combined. Be careful not to overmix; a few streaks of flour are fine. Fold in the chocolate chips, if using.

Pour the brownie batter into the prepared baking pan and spread evenly. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with moist crumbs, but not wet batter. The edges should look set.

Remove the pan from the oven and let the brownies cool completely in the pan on a wire rack before lifting them out using the parchment paper overhang. This is crucial for fudgy, neat squares. Once completely cool, cut into 16 squares and serve.