1. Heat oven to 350°F.
2. Beat butter and 3/4 cup peanut butter in large bowl until well blended; beat in brown sugar. Add eggs and vanilla, beating until well blended.
3. Stir together oats, flour, baking soda and salt; gradually stir into butter mixture. (Dough will be thick.) Set aside 2 cups dough.
4. Press remaining dough onto bottom of ungreased 15-1/2x10-1/2x1-inch jelly-roll pan. Prepare CHOCOLATE PEANUT BUTTER FILLING; spread over dough. Set aside 1/2 cup peanut butter chips for drizzle. Add remaining peanut butter chips to reserved dough; crumble mixture over filling.
5. Bake 25 minutes or until top is golden brown (chocolate will be soft). Cool completely in pan on wire rack.
6. Place reserved 1/2 cup peanut butter chips and shortening in small microwave-safe bowl. Microwave at MEDIUM (50%) 30 seconds; stir. If necessary, microwave an additional 10 seconds at a time, stirring after each heating, until chips are melted and mixture is smooth when stirred. Drizzle over bars; allow drizzle to set. Cut into bars. About 4 dozen bars.
CHOCOLATE PEANUT BUTTER FILLING
1/2 cup REESE'S Creamy Peanut Butter
1/3 cup butter or margarine
1/2 cup HERSHEY'S Cocoa
1 can (14 oz.) sweetened condensed milk (not evaporated milk)
1-1/2 teaspoons vanilla extract
Melt peanut butter and butter in medium saucepan over low heat. Stir in cocoa. Add sweetened condensed milk; heat, stirring constantly, until smooth and thick. Remove from heat. Stir in vanilla.