Combine first 4 ingredients in a large bowl; stir well. Combine flour, oats, baking powder, baking soda, and salt. Add to butter mixture, stirring just until blended. Fold in chopped candy bars.
Spoon batter into a greased aluminum foil-lined 13" x 9" pan coated with cooking spray, allowing foil to extend over ends of pan. (Pan will be very full.) Spoon dollops of dulce de leche over batter; swirl slightly into batter with a knife. Bake at 325° for 1 hour and 5 minutes. Remove to a wire rack, and cool completely. (This may take several hours.) Use foil to lift uncut brownies out of pan. Peel foil away from sides of uncut brownies, and cut into bars.
*Find dulce de leche with other Mexican ingredients or on the baking aisle.