Preheat oven to 350°. Spray a 15x10-inch metal jelly roll pan with baking spray with flour.
In a medium saucepan, whisk butter, 1 cup water, and cocoa powder over medium-low heat until butter is melted and mixture is smooth. Pour into a bowl; let cool until slightly warm.
In a large bowl, combine flour, brown sugar, espresso, baking soda, and salt.
In a small bowl, whisk together buttermilk, eggs, and vanilla. Make a well in the center of dry ingredients; add butter mixture and egg mixture, whisking until smooth. Pour into prepared pan.
Bake until a wooden pick inserted in center comes out clean, about 15 minutes. Let cool until slightly warm, about 30 minutes. Spread Chocolate Icing over cake. Sprinkle with pecans. Let stand until frosting is set.
Chocolate Icing
In a small saucepan, whisk ⅓ cup water, butter, and cocoa powder over medium heat until butter is melted and mixture simmers. Remove from heat.
In a large bowl, whisk together butter mixture, confectioners’ sugar, vanilla, and salt until smooth. Immediately spread over slightly warm cake.
Recipe by Taste of the South at https://tasteofthesouthmagazine.com/texas-sheet-cake-2/