Preheat oven to 350 degrees. Coat a cookie sheet or jelly roll pan with non-stick spray. Set aside.
In a medium-size pan, add butter, water, and cocoa powder and bring to a boil. Remove from heat and let it cool as you assemble your dry ingredients.
In a large bowl, whisk together flour, sugar, baking soda, and salt. Then slowly whisk in the cocoa mixture until smooth. Add sour cream, stirring until smooth.
Pour batter into prepared pan, spreading batter evenly with a knife. Bake for 15-18 minutes, or until a toothpick inserted into the center comes out clean.
For the Peanut Butter Icing: In a large bowl, stir together powdered sugar, milk, melted butter, and vanilla until smooth. Stir in peanut butter until smooth. If frosting is too thick, microwave for 30 seconds or add a tablespoon of milk.
Pour over warm cake then spread evenly. Cool completely and serve.