Heat oven to 350. Spray a rimmed 10" x 15" pan w/ cooking spray. In saucepan, combine 1 cup butter, water and 1/4 cup cocoa powder. Cook over low until butter is melted, stirring occasionally.
Remove pan from heat, pour mix into lg bowl. Allow to cool. Add to bowl, the flours, sugar, eggs, sour cream, baking soda and salt. Beat until combined. Pour into baking sheet. Bake 25 mins, or until a toothpick inserted in center comes out clean. let cool.
In small saucepan, over low heat, melt 1/2 stick butter w/ milk and remaining cocoa powder. Remove from heat. Stir in powdered sugar and vanilla, mix well. Frost cake w/ mix. Sprinkle top of cake w/ candy pieces.