Step 1: Preheat Oven

1. Preheat Oven: Preheat your oven to 350°F (175°C). Grease and flour a 18×13-inch sheet pan.

Step 2: Prepare the Cake Batter

1. Mix Dry Ingredients: In a large mixing bowl, whisk together the flour, sugar, baking soda, and salt.
2. Heat Butter and Cocoa: In a saucepan, combine the butter, water, and cocoa powder. Bring to a boil over medium heat, stirring frequently.
3. Combine Wet and Dry: Pour the hot cocoa mixture into the dry ingredients and mix until combined.
4. Add Eggs and Sour Cream: Add the eggs, vanilla extract, and sour cream to the batter. Mix until smooth and well combined.

Step 3: Bake the Cake

1. Pour Batter: Pour the batter into the prepared sheet pan and spread evenly.
2. Bake: Bake in the preheated oven for 20–25 minutes, or until a toothpick inserted in the center comes out clean.

Step 4: Prepare the Frosting

1. Make Frosting: While the cake is baking, prepare the frosting. In a saucepan, melt the butter over medium heat. Stir in the cocoa powder and milk, bringing the mixture to a boil.
2. Add Powdered Sugar: Remove from heat and whisk in the powdered sugar and vanilla extract until smooth and glossy. If using, stir in the chopped nuts.

Step 5: Frost the Cake

1. Frost While Warm: Once the cake is done baking, remove it from the oven and pour the warm frosting over the hot cake, spreading it evenly.

Step 6: Serve

1. Cool and Slice: Allow the cake to cool for a bit before slicing into squares. Serve warm or at room temperature.