Preheat the oven to 350 degrees F. Grease a 9” x 13” baking dish with 1 tablespoon of the butter. Sprinkle 1 tablespoon of flour into the pan to coat the sides and bottom, knocking out any excess. Set aside.
In the top of a double boiler or in a bowl set over simmering water, combine the remaining 12 tablespoons butter and the chocolate. Cook, stirring, until smooth, about 3 minutes. Add the sugar and whisk until smooth. Add 1/4 cup of the flour and whisk to combine. Alternate adding the remaining flour and eggs, whisking well after each addition. Stir in the toasted nuts and vanilla.
For the Fudge Topping: While the brownies are in the oven, in a medium saucepan, combine the butter, cocoa powder and buttermilk over medium heat. Cook until the butter melts and mixture just comes together. Remove from the heat and whisk until smooth. Sift the confectioners' sugar into a medium bowl. Add the chocolate mixture and vanilla and whisk until smooth.