Preheat your oven to 350°F (175°C). Grease a 10x15-inch jelly roll pan or similar-sized baking sheet. In a large bowl, whisk together the flour, sugar, baking soda, and salt. Set aside.

In a medium saucepan, combine the butter, water, and cocoa powder. Heat over medium heat, stirring constantly, until the butter melts and the mixture comes to a gentle boil. Remove from heat and pour this hot mixture over the dry ingredients. Stir until well combined.

In a small bowl, whisk together the eggs, sour cream, and vanilla extract. Add this to the chocolate mixture and stir until smooth and well incorporated. The batter will be thin - this is normal!

Pour the batter into the prepared baking sheet and spread it evenly. Bake for 18-22 minutes, or until a toothpick inserted into the center comes out clean. While the cake bakes, prepare the frosting.

In a medium saucepan, combine the butter, cocoa powder, and milk. Heat over medium heat, stirring constantly, until the butter melts and the mixture comes to a boil. Remove from heat and whisk in the vanilla extract. Gradually add the powdered sugar, whisking until smooth. Stir in the nuts if using.

As soon as the cake comes out of the oven, pour the warm frosting over the hot cake. Use a spatula to spread it evenly across the surface. The frosting will set as the cake cools. Allow to cool completely before cutting into squares.