Preheat the oven to 350F. Grease a 18x13 inchbaking sheet.
Add butter, cocoa powder, and butter to a medium sauce pot and bring to a boil over medium-high heat.
In a medium sized mixing bowl, combine flour, sugar, baking soda, and salt. Set aside.
In a small mixing bowl, whisk together sour cream, eggs, and vanilla. Set aside.
Add egg mixture to the flour mix and stir to combine.
When cocoa mixture is boiling, add it to the flour and egg mix and stir to combine. Be sure to mix thoroughly so ensure no lumps in the batter.
Pour batter into the prepared baking sheet and smooth with a spatula.
Bake for 15-18 minutes. While the cake bakes, prepare the frosting.
Add the milk, cocoa, and butter to a medium sauce pot and bring to a boil.
Once it boils, remove from the heat and stir in the powdered sugar. Whisk to combine making sure no lumps are present. Pour the hot icing over the hot cake using a small spatula to smooth the icing over the entire surface. Let the cake cool for 10-15 minutes before slicing.