Preheat oven to 350 degrees. Line a 9x9 pan with parchment paper.
Place the chopped chocolate in a microwave safe bowl. Heat on 50% power for 2 minutes. Stir until all pieces are melted. Set aside to cool.
In a large bowl, combine flour, cocoa powder, baking powder and salt. Stir together until well combined. Set aside.
In the bowl of an electric mixer, combine the sugar and oil. Beat on medium high speed for 2-3 minutes. Scrape down the bowl. Add the eggs, one at a time, beating for about 60 seconds between each addition. Scrape down the bowl. Add the vanilla, sweet potato and melted chocolate, mix on medium speed for 1-2 minutes. Turn the mixer to the lowest speed and alternately add the milk and flour mixture. Begin and end with the milk.
Pour the batter into the prepared pan. Bake for 40-45 minutes, until the top springs back when gently poked. This is a very moist cake, so be careful not to under bake as the center will sink.
Remove from the oven and cool for 15 minutes in the pan. The remove the cake from the pan and cool on a wire cooling rack. Store leftovers in an airtight container.
Notes
Top with powdered sugar, whipped cream, chocolate ganache or even buttercream. This makes a great moist chocolate cake!