Combine butter, water and cocoa in a saucepan and bring to a boil.
Whisk the flour, sugar, and salt in a large mixing bowl until combined. Add the cocoa mixture into the flour mixture and whisk until a batter is formed. Add the sour cream, eggs, and baking soda and stir until combined.
Spread the batter onto a greased sheet pan and bake at 375°F for 20-25 minutes.
For the Icing
Sift the cocoa powder and powdered sugar into the stand mixer bowl.
Heat the butter and evaporated milk in a saucepan just until it comes to a gentle boil. Remove the pan from the heat and stir in the vanilla and pinch of salt.
Fit the whisk attachment onto the stand mixer and carefully poor the heated butter mixture into the powdered sugar mixture while on low speed.
Increase the speed to medium high and mix the icing until it's well combined, scraping the sides down occasionally to make sure it's well mixed.
Ice the cake while still warm. I find that if I start the icing right when the cake comes out of the oven, it's ready to be frosted as soon as the icing comes together.
Wait for the cake to cool before cutting.All ovens vary. Please watch your cake starting at 20 minutes!