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 size mixing bowl, whisk together the flour, cocoa powder, baking powder, baking soda and salt. 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 fitted with the paddle attachment, beat the oil, sugar and eggs on medium speed for about two minutes, until light and fluffy in color and texture.
Add the melted baking chocolate and stir to combine. Scrape down the sides and bottom of the bowl. Mix for another 20 to 30 seconds.
With the mixer on low speed, add 1/3 of the dry ingredients, followed by half of the buttermilk and sour cream mixture, 1/3 of the dry ingredients, the second half of the buttercream and sour cream mixture and then the final 1/3 of the dry ingredients. Mix just until combined.
Pour the batter into the prepared pan and bake for about 25 to 30 minutes, or until a toothpick inserted in the center of the cake comes outs with just a few moist crumbs on it.
Let cool completely before frosting.
For the Chocolate Sour Cream Buttercream
In the bowl of a stand mixer fitted with the paddle attachment, beat the butter for about 2 minutes on medium-high speed.
Turn the mixer to low speed and slowly add the melted chocolate. Scrape down the sides of the bowl and mix well.
Gradually add the powdered sugar, vanilla and pinch of salt. Add the sour cream.
Turn the mixer to medium-high speed and beat the frosting for about 5 minutes.
Evenly spread over the cake layer once the cake has cooled completely.
NOTE: This cake can be made into a two layer 8-inch round cake by doubling the batter and buttercream.