Preheat your oven to 350°F (175°C) to ensure even baking.
Grease a 13x18 inch baking sheet with butter or non-stick spray to prevent sticking.
In a large mixing bowl, combine the flour, sugar, baking soda, and salt.
In a saucepan, melt the unsalted butter, vegetable shortening, cocoa powder, and water together over medium heat, stirring until smooth.
Pour the melted cocoa mixture into the bowl with the dry ingredients. Add the buttermilk, eggs, vanilla extract, and milk. Mix well until the batter is smooth and well combined.
Pour the batter into the prepared baking sheet and spread it evenly. Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
While the cake bakes, prepare the frosting by melting the unsalted butter and cocoa powder together over medium heat in a saucepan. Remove from heat and stir in powdered sugar, milk, vanilla extract, and a dash of salt until smooth.
Remove the cake from the oven and let it cool for about 15 minutes. Pour the warm frosting over the cake and spread evenly. Sprinkle chopped pecans or walnuts on top.
Allow the cake to cool completely on a wire rack before slicing into squares and serving. Enjoy!