Preparation Steps

• Preheat oven to 350°F (175°C). Grease a 15x10x1-inch jelly roll pan or large sheet cake pan.
• In a large bowl, whisk together flour, sugar, baking soda, and salt. Set aside.
• In a medium saucepan, combine butter, water, and cocoa powder. Heat over medium heat, stirring until butter melts and mixture is smooth. Bring to a boil, then remove from heat.
• Pour the hot chocolate mixture over the dry ingredients and stir until combined.
• In a small bowl, whisk together buttermilk, eggs, and vanilla. Add to the batter and mix until smooth.
• Pour batter into the prepared pan and spread evenly.
• Bake for 20–25 minutes, or until a toothpick inserted in the center comes out clean.
• While the cake bakes, prepare the frosting: In a saucepan, combine butter, cocoa powder, and milk. Heat over medium heat until butter melts and mixture comes to a boil.
• Remove from heat and stir in vanilla and powdered sugar until smooth. Stir in pecans, if using.
• Immediately pour the warm frosting over the hot cake as soon as it comes out of the oven. Spread evenly.
• Allow cake to cool completely before cutting into squares.