Preheat oven to 350°F (175°C). Grease a 13x9x2-inch (25x38x4 cm) baking pan. In the bowl of your mixer, whisk together flour, sugar, baking soda and salt.
In a medium saucepan combine butter, cocoa powder and water. Bring the mixture just to boiling over medium-high heat, stirring constantly. Remove from heat and pour the chocolate mixture into the flour mixture. Beat with mixer on medium speed until combined. Scrape bowl and add buttermilk, eggs and vanilla extract or vanilla sugar. Beat for one minute on medium speed. The batter will be thin. Pour into prepared pan.
Bake at 350°F (175°C) for 20-25 minutes, until a toothpick inserted near the center of brownies comes out clean. Cool in the pan on a wire rack.
Chocolate-Buttermilk Icing
Begin making the chocolate-buttermilk icing immediately when the brownies are done baking. Place the powdered sugar into the bowl of your mixer. In a medium saucepan bring butter, unsweetened cocoa powder and buttermilk to a boil over medium heat, stirring constantly. This mixture will seem oily and separated but that's normal. Remove from heat and pour chocolate mixture over powdered sugar and add vanilla extract or vanilla sugar. Beat until smooth, scraping bowl once. Working quickly, distribute the icing over the warm brownies and then spread the icing evenly.
Cool the brownies completely in the pan on a wire rack and cover and chill them in the refrigerator before cutting. Keep brownies covered in the refrigerator for up to three days.