Preheat oven to 350 F (180 C). Spray a 17¼ x 12¼ -inch rimmed baking sheet with baking spray with flour. Line bottom of pan with parchment paper.

In a medium saucepan, heat butter, 1 cup (240 grams) water, and cocoa over medium-low heat, whisking occasionally, until butter is melted and mixture is smooth. Pour into a bowl; let cool until slightly warm.

In a large bowl, whisk together flour, brown sugar, espresso powder, baking soda, and salt. Make a well in center.

In a small bowl, whisk together buttermilk, eggs, and vanilla. Add buttermilk mixture to chocolate mixture, and whisk until combined. Add chocolate mixture to flour mixture, whisking until smooth and stopping to scrape sides of bowl. Pour into prepared pan, and spread into an even layer. Tap pan on a kitchen towel-lined counter to release any air bubbles.

Bake until cake pulls away from sides of pan and a wooden pick inserted in center comes out clean, 12 to 15 minutes. Let cool completely in pan on a wire rack.

Freeze cooled cake until firm, about 30 minutes.

Invert frozen cake onto a cutting board, and discard parchment. Trim edges to create a 16x11-inch cake. Cut into thirds crosswise to create 3 (11x5⅓-inch) rectangles. Place1 rectangle, top side up, on a cake plate. Spread 1 cup (270 grams) Chocolate Frosting on top, and sprinkle with one-third of pecans. Top with second rectangle; spread1 cup (270 grams) frosting on top, and sprinkle with one-third of pecans. Top with remaining rectangle; spread remaining Chocolate Frosting on top, and sprinkle with remaining pecans. Serve immediately, or cover and refrigerate until ready to serve. Serve at room temperature.