1Heat oven to 350°F. Grease and flour a 15x10x1-inch jelly roll pan.
2Combine water, butter and cocoa in small saucepan. Cook over medium heat, stirring occasionally, until mixture boils; boil and stir for 1 minute. Remove from heat and set aside.
3Stir together flour, sugar, baking soda and salt in large bowl. Add eggs and sour cream; beat until well blended. Add cocoa mixture; beat just until blended (batter will be thin). Pour into prepared pan.
4Bake 25-30 minutes or until wooden pick inserted in center comes out clean. Do not remove cake from pan. Spread peanut butter over warm cake. Cool completely in pan on wire rack.
5Prepare CHOCOLATE TOPPING:
Place 2 cups chocolate chips and the shortening in small microwaveable bowl. Microwave at MEDIUM (50%) power for 1½ minutes, stir. If necessary, microwave at MEDIUM an additional 15 seconds at a time, stirring after each heating, just until chips are melted when stirred.
6Carefully spread chocolate topping over top of cake, covering peanut butter. Allow topping to set, then cut into squares.