Combine water, butter and unsweetened chocolate in a medium saucepan. Over medium heat bring to boil and boil for one minute, remove from heat and let cool.
Combine flour, sugar, baking soda and salt and add to cooled chocolate mixture. Add eggs, sour cream and vanilla, mix until well blended. Stir in chopped walnuts.
Pour into a greased and floured brownie pan, bake approximately 35 minutes.
Melt butter and unsweetened chocolate together over low to medium heat. Once melted and smooth remove from heat and using a mixer alternately add powdered sugar, milk and vanilla and blend until smooth. Spread over warm brownies.