Preheat your oven to 350°F. Grease an 18x13-inch baking sheet and set it aside.
In a large mixing bowl, whisk together the flour, sugar, cocoa powder, baking soda, and salt. Next, in a separate bowl, combine eggs, sour cream, warm water, and vanilla extract. Gradually mix the wet ingredients into the dry ingredients until a smooth batter forms.
Pour the batter into the prepared baking sheet, ensuring an even spread. Bake in the preheated oven for 15-20 minutes or until a toothpick inserted into the center comes out clean. Allow the cake to cool slightly.
In a medium saucepan, melt the butter over low heat. Add cocoa powder and milk, stirring until smooth. Remove from heat and gradually whisk in powdered sugar until a smooth frosting forms.
Pour the frosting over the slightly cooled cake, spreading it evenly. If you're using them, sprinkle the chopped pecans or walnuts on top. Allow the cake to cool completely before slicing and serving.