1. Heat oven to 350°F. Beat butter, peanut butter and brown sugar until well blended. Add eggs and vanilla; beat thoroughly.
2. Stir together oats, flour, baking soda and salt; gradually add to butter mixture. (Dough will be thick.) Remove 2 cups dough; set aside. Press remaining dough onto bottom of 13x9x2-inch baking pan.
3. Prepare CHOCOLATE FILLING. Spread filling evenly over dough. Sprinkle candy pieces evenly over filling. Crumble reserved dough evenly over filling.
4. Bake 25 minutes or until top is golden brown. (Chocolate will be soft.) Cool completely in pan on wire rack; cut into bars. Makes 24 bars.
Melt 1/2 cup (1 stick) butter or margarine in saucepan over low heat. Stir in 2/3 cup HERSHEY'S Cocoa and 1/3 cup sugar. Add 1 can (14 oz.) sweetened condensed milk; cook, stirring constantly, until smooth and thick. Remove from heat; stir in 1-1/2 teaspoons vanilla extract.