Preheat your oven to 350℉.
Butter the bottom and sides of an 8" round cake pan. Line the bottom with parchment paper and butter the parchment. Set aside.
In the bowl of a stand mixer, sift the flours, baking soda and baking powder together.
Add the salt and the sugar and beat on low with the paddle attachment for 30 seconds or until combined.
Add the room temperature butter and beat on low speed until the mixture looks like sand, about 2-3 minutes.
In a separate bowl whisk together the oil, eggs, vanilla extract and buttermilk.
Add the wet ingredients to the butter/flour mixture and beat on medium speed for 2 minutes until the mixture is smooth.
Diivide the batter evenly into 2 bowls.
Add the melted and cooled chocolate and the cocoa powder to one portion of the batter and whisk until incorporated and the batter is smooth.
Add the 1 tablespoon of all-purpose flour to the other half and whisk until incotporated and the batter is smooth.
Randomly dollp the 2 batters into the prepared pan and swirl using a flat knife.
Bake for 38-40 minutes or until the top springs back when gently pressed or a toothpick inserted in the center comes out clean.
Remove the pan from the oven and place it on a rack to cool for 15 minutes.
Remove the cake from the pan and place it on a rack to cool completely.
