Combine first 3 ingredients in a large bowl; cut in cold butter pieces with a pastry blender or 2 forks until crumbly. Press mixture onto bottom of a 13- x 9-inch pan.
Bake at 350° for 15 minutes. Remove pan, and place on a wire rack.
Stir together condensed milk, vanilla, and 1 cup chocolate morsels in a small saucepan over medium heat, stirring occasionally, 5 minutes or until smooth. Pour mixture evenly over prepared crust, spreading to edges of the pan. Sprinkle with chopped pecans or walnuts and remaining chocolate morsels.
Bake at 350° for 20 minutes. Cool in pan on a wire rack 1 hour or until completely cool. Cut into 48 (1 1/2-inch) bars.
Note: Store bars in airtight containers up to 1 week. You may also freeze bars up to 1 month.