Preheat your oven to 325 degrees F. Spray an 8" x 8" square baking pan with nonstick spray, line the bottom with parchment paper and spray again. Set aside.
In a medium sized mixing bowl, combine the flour, cocoa powder, baking powder, baking soda and salt. Whisk to combine. Set aside.
In a small bowl, combine the sour cream and the buttermilk. Stir to combine. Set aside.
In the bowl of a stand mixer, or using a hand mixer, beat the oil, sugar and eggs on medium speed for about two minutes.
Add the melted baking chocolate and stir to combine.
With the mixer on low speed, alternately add the dry ingredients and the wet ingredients to the oil and sugar mixture. Start and end with the dry ingredients and mix only until incorporated.
Pour the batter into the prepared pan and bake at 325 degrees F. for about 40 minutes. The cake is done when a toothpick is inserted into the center of the cake and comes out with a few moist crumbs on it. Let cool completely before adding frosting.