Combine first 3 ingredients in a medium bowl. Cut in margarine with a pastry blender until mixture resembles coarse crumbs. Stir in egg and vanilla.
Pat dough into a 13- x 9- x 2-inch pan coated with cooking spray. Bake at 375° for 10 minutes. Remove from oven; let cool completely on a wire rack.
Combine chocolate morsels and peanut butter in a small saucepan. Cook over medium heat, stirring constantly, until mixture melts. Spread chocolate mixture over crust; cover and chill at least 1 hour. Cut into 24 bars. Store in refrigerator in an airtight container.