Heat oven to 350°F.
Stir together flour, oats, baking soda and salt. Beat 1/2 cup (1 stick) butter, peanut butter and brown sugar until well blended in large bowl. Add eggs and 1 teaspoon vanilla; beat until light and fluffy. With spoon, gradually add flour mixture. (Dough will be thick.) Remove 2 cups dough; set aside. Press remaining dough evenly onto bottom of ungreased 15-1/2x10-1/2x1-inch jelly-roll pan.
Melt remaining 1/2 cup (1 stick) butter in medium saucepan over low heat. Stir in cocoa and granulated sugar. Add sweetened condensed milk; cook, stirring constantly, until smooth and thick. Remove from heat; stir in remaining 1-1/2 teaspoons vanilla. Spread mixture evenly over dough. Sprinkle top of chocolate with peanut butter baking bits. Crumble reserved dough evenly over chocolate and baking bits.
Bake 25 to 30 minutes or until top is golden brown (chocolate will be soft). Cool completely in pan on wire rack; cut into bars. About 4 dozen bars.