Preheat oven to 350 degrees. Prepare a sheet cake pan with non stick cooking spray.
In a large (heat proof) mixing bowl whisk together your flour, sugar, baking soda, and salt. Set aside for later.
Melt butter with unsweetened cocoa powder and water in a sauce pan. Bring to a boil then remove from heat.
In a separate bowl whisk together sour cream with eggs and vanilla extract. Add this to your flour mix until well combined. Next add in your melted butter from the sauce pan to your flour and mix again until well combined.
Pour this batter into your prepared sheet cake pan. Bake for 15-20 minutes your until your cake is cooked through.
Melt butter, milk, and unsweetened cocoa powder. Bring to a boil then remove from heat. Add in powdered sugar and mix until well combined. Add your frosting to the cake right away then allow it to cook for 10-15 minutes before serving.