1Combine first 3 ingredients in a large bowl; mix well, and set aside.
Combine water, butter and cocoa in a heavy saucepan; bring to a boil, stirring constantly.
Gradually stir hot mixture into flour mixture; stir well.
Stir in eggs and sour cream; mix well.
Pour into a greased and floured 13x9x2-inch baking pan.
Bake at 350 F for 25-30 minutes or until a toothpick inserted in the center comes out clean. Cool.
Combine butter, milk, and cocoa in a saucepan; bring to a boil, stirring constantly.
Remove from heat. Add remaining ingredients; beat at medium speed of an electric mixer until smooth.
Frost cake with Chocolate-Peanut Butter Frosting.