Preheat the oven to 350F
In a small saucepan combine the butter, water and cocoa over medium heat just until the butter melts being careful to not allow it to cook too long.. mix constantly.
In a large mixing bowl combine sugar, flour, salt, eggs, and baking soda.
Add in the butter mixture to the dry ingredients, then add the sour cream and vanilla and mix until completely incorporated
Pour the cake batter into a sheet cake pan and bake for 18-20 minutes or until completely set.