Step 1: Preheat your oven to 350°F (175°C) to ensure even baking.
Step 2: In a medium saucepan, combine 1 cup of unsalted butter, 1 cup of water, and 5 tablespoons of cocoa powder. Heat over medium heat until melted and smooth.
Step 3: In a large mixing bowl, whisk together 2 cups of flour, 2 cups of sugar, 1 teaspoon of baking soda, and 0.5 teaspoon of salt.
Step 4: Pour the warm chocolate mixture into the dry ingredients and stir until just combined.
Step 5: Add 2 eggs, 0.5 cup of sour cream, and 1 teaspoon of vanilla extract. Mix until smooth.
Step 6: Pour the batter into a greased baking sheet and bake for 20-25 minutes.
Step 7: For the frosting, melt 0.5 cup of butter, stir in 6 tablespoons of milk and 5 tablespoons of cocoa powder, then simmer and remove from heat.
Step 8: Whisk together 4 cups of powdered sugar and 1 teaspoon of vanilla extract in a bowl, then pour the butter mixture over and mix until smooth.
Step 9: Once the cake cools for about 10 minutes, pour the frosting over and spread evenly. Optionally, sprinkle with chopped nuts.
