Preheat the oven to 350°F and lightly grease an 18x13-inch jelly roll pan.

In a medium saucepan over medium heat, combine water, butter, and cocoa powder. Stir occasionally and bring the mixture to a boil.

In a large bowl, whisk together the flour, sugar, baking soda, and salt.

In a separate bowl, mix the sour cream, eggs, and vanilla extract until smooth. Incorporate this mixture into the dry ingredients and stir until well combined.

Once the chocolate mixture reaches a boil, remove it from the heat and carefully pour it into the batter. Mix thoroughly until smooth and free of lumps.

Pour the batter evenly into the prepared pan, smoothing the surface with a spatula. Bake for 15–20 minutes, or until the cake is set.

While the cake bakes, prepare the frosting.

In a medium saucepan, combine milk, cocoa powder, and butter. Bring to a boil, stirring frequently. Remove the saucepan from the heat and whisk in the powdered sugar until the frosting is smooth. Use a hand mixer if necessary to eliminate lumps.

Once the cake has finished baking, pour the hot frosting over the warm cake. Spread it evenly with a spatula. Allow the frosting to set for about 10 minutes before serving.