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.