Preheat oven to 375 degrees Fahrenheit and spray a 13"x9" baking dish with non-stick spray. Set aside.
In a large saucepan over medium-low heat, add butter, water, and baking chocolate. Stir until chocolate has melted. Remove from heat and allow to cool for 5 minutes.
To the same saucepan with the melted chocolate, add flour, brown sugar, baking soda, and salt and mix until well combined. Add in vanilla, eggs, and sour cream and stir well.
Pour brownie batter into your prepared baking pan and bake for 30 minutes.
Cool on a wire rack for 20 minutes.
While the cake is baking, take a small saucepan and add butter, baking chocolate, and milk. Bring mixture to simmer, while stirring constantly. Remove from heat and add in powdered sugar and vanilla.
Spread icing over warm brownies and allow the cake to cool completely before cutting into bars. Enjoy!