How to Make It
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.