This incredibly moist cake is Texas-sized in both its dimensions and its deep chocolate flavor. Slideshow: More Cake Recipes
Preheat the oven to 375°. Generously butter an 18-by-13-inch rimmed baking sheet. In a large bowl, whisk the flour with the sugars, baking soda and 1/2 teaspoon of the kosher salt. On a second rimmed baking sheet, spread the pecans in a single layer. Bake for 8 minutes, until toasted and fragrant. Let cool completely and coarsely chop.
In a medium saucepan, melt 1 stick of the butter over moderate heat. Stir in the coffee and 1/2 cup of the cocoa powder and bring to a boil, stirring occasionally, about 10 minutes. Pour the warm cocoa mixture into the dry ingredients and stir until only a few streaks of white remain. Add the eggs, sour cream and 1/2 teaspoon of the vanilla and stir until the batter is smooth and no streaks remain.
Scrape the batter into the prepared baking pan and smooth the top with a spatula. Bake for 15 minutes, until the cake springs back lightly when touched and a toothpick inserted into the center of the cake comes out clean. Transfer to a wire rack to cool.
In a medium saucepan, melt the remaining 1 stick of butter over moderate heat with the remaining 1/4 cup of the cocoa powder and the buttermilk. Bring the mixture to a boil, whisking frequently. Remove the pan from the heat and whisk in the confectioners’ sugar and 1/2 teaspoon each of vanilla and salt. Pour the icing over the warm cake and use an offset spatula or a knife to spread evenly. Sprinkle the toasted pecans over the cake and let stand until cool, about 30 minutes. Slice into squares and serve.
The cake can be kept at room temperature wrapped in plastic for 2 days.