Preheat your oven to 350°F (175°C) and prepare an 18x13-inch sheet pan by greasing it with butter or lining it with parchment paper.

In a large mixing bowl, whisk together 2 cups of all-purpose flour, 2 cups of granulated sugar, 1 teaspoon of baking soda, and ½ teaspoon of salt until combined.

In a medium saucepan over medium heat, combine 1 cup of unsalted butter, 1 cup of water, and ½ cup of unsweetened cocoa powder. Stir continuously until the mixture comes to a boil.

Once the cocoa mixture reaches a rolling boil, carefully pour it into the bowl with the dry ingredients. Stir well using a whisk or spatula until fully combined.

Incorporate 2 large eggs, 1 teaspoon of vanilla extract, and 1 cup of sour cream into the batter mixture. Mix gently until the batter is smooth and well combined.

Pour the smooth cake batter into your prepared sheet pan, spreading it evenly with a spatula.

Place the sheet pan in the preheated oven and bake for 20-25 minutes. Check for doneness by inserting a toothpick into the center; it should come out clean.

While the cake is baking, prepare the frosting by melting ½ cup of unsalted butter in a saucepan. Stir in ½ cup of unsweetened cocoa powder and ⅓ cup of milk, bringing the mixture to a boil.

Remove the frosting mixture from heat and whisk in 4 cups of powdered sugar and 1 teaspoon of vanilla extract until smooth and glossy.

Once the cake is baked and slightly cooled, pour the warm frosting over the top. Spread it evenly with a spatula.

Allow the frosted cake to cool in the pan for about 10-15 minutes before slicing it into squares.