Preheat oven to 350 degrees . In large saucepan, melt 11/2 sticks butter, chocolate, granulated sugar, 1/3 cup cocoa and 1 cup water over medium, whisking until smooth and warm. Let cool, 10 minutes. Grease bottom and sides of 9-by-13-inch metal baking pan. Whisk buttermilk, eggs, 1 tsp. vanilla and 1/2 tsp. salt into chocolate mixture. Add flour, baking powder and baking soda; whisk just until blended. Transfer batter to prepared pan; bake until springy in center, about 45 minutes. Make frosting: In medium saucepan, melt remaining 4 tbsp. butter over medium heat. Whisk in 6 tbsp. milk and remaining 1/4 cup cocoa, 1 tsp. vanilla and 1/4 tsp. salt; remove from heat. Whisk in confectioners sugar until smooth (mix in 1 to 2 tbsp. more milk if too thick). Let cake cool in pan on rack for 15 minutes, then frost warm cake. Sprinkle with pecans. Let cake cool completely before cutting and serving.
The Do's and Don'ts of Cooking with Chocolate