Grease the bottom and sides of a 17x11" cookie sheet pan.
In a saucepan, mix the butter, water and cocoa powder. Bring to a low boil.
Pour the chocolate mixture into a large bowl. Set aside.
In a small bowl, mix together the sugar, flour and salt.
Gradually add the dry mixture to the chocolate and mix well.
Add the eggs and mix well.
Add the sour cream and baking soda. Mix well.
Pour into the greased cookie sheet and bake for 20 minutes.
For the Frosting:
*Plan to make the frosting about 15 minutes before the cake is done. The cake should be frosted while it is warm.
Melt the butter in a saucepan over medium low heat.
Add the cocoa powder and milk. Heat just until the mixture starts to bubble. Remove from the heat.
Using a hand mixer, gradually beat in the powdered sugar and vanilla until completely blended.
Spread the frosting on the cake as soon as it is out of the oven.
Allow the cake to cool completely.
Store any leftovers in an airtight container.
The calories shown are based on the cake being cut into 20 pieces, with 1 serving being 1 slice of frosted cake. Since different brands of ingredients have different nutritional information, the calories shown are just an estimate.