Cake: Preheat oven to 350 degrees. Lightly oil a 12-cup Bundt pan or coat it with non-stick cooking spray. Dust the pan with flour, shaking out excess. In a large mixing bowl, whisk together flour, granulated sugar, cocoa powder, baking soda, baking powder and salt. Add buttermilk, brown sugar, eggs, oil and vanilla. Beat with an electric mixer on medium speed for 2 minutes. Whisk in hot coffee until completely incorporated. The batter will be quite thin. Pour the batter into the prepared pan. Bake for 35 to 40 minutes, or until cake tester inserted in center comes out clean. Cool the cake for 10 minutes. Remove from pan and let cool completely on rack. Icing: In a small bowl, whisk together confectioner's sugar, vanilla and enough of the buttermilk to make a thick but pourable icing. Set the cake on a serving platter and drizzle the icing over the top.