How to Make It
Preheat oven to 325°F. Grease and line a metal 9-by-13-inch baking dish with parchment paper; set aside. In a medium saucepan, melt butter until boiling. Immediately add chocolate chips and remove from heat. Stir once to coat chocolate chips in butter, and allow to sit for 5 minutes.
In a large mixing bowl, whisk together sweet potato, brown sugar, granulated sugar, eggs, vanilla, and salt. Whisk chocolate and butter mixture until chocolate is fully melted and combined with butter. Add chocolate mixture to sweet potato mixture and whisk to incorporate.
Sift flour and cocoa into the bowl with the liquid ingredients, and stir until no pockets of dry ingredients remain. Pour into prepared baking dish, level batter with spatula, and bake until a toothpick inserted comes out clean and edges have set, about 35 minutes. Allow to cool completely before slicing and dusting with powdered sugar, if desired.
To make the mashed sweet potato, wrap sweet potatoes in aluminum foil and roast in a 425°F oven for 1 1/2 hours. Turn off the oven and allow the sweet potatoes to cool in the oven for 2 hours. This allows more of the starches in the sweet potato to transform into complex sugars, making the sweet potato flesh both sweeter and softer. Peel and mash the sweet potatoes.