Preheat the oven to 350 degrees F. Prepare a 10-inch Bundt pan with a light coating of oil and set aside.
In a saucepan, combine the chocolate and oil on medium-low heat, stirring until thoroughly combined. Transfer to a bowl and set aside.
In another bowl, sift together the flour, cocoa, salt, baking soda, and baking powder. Beat the brown sugar into the melted chocolate until well blended. Add the egg and egg whites, one at a time, beating well after each addition. Stir in the vanilla. Alternate adding about 1/4 cup of the buttermilk and 1/3 of the flour mixture, beating well after each addition.
Pour the batter into the prepared pan and bake for about 35 minutes, or until the cake pulls away from the sides of the pan and a knife inserted in the center comes out clean.
Allow the cake to cool in the pan for 5 to 10 minutes, then invert it onto a plate. Serve warm, or cool completely.