1 cup butter or margarine, softened
2 cups packed brown sugar
2 eggs
2 teaspoons vanilla
2½ cups all-purpose flour
1 teaspoon baking soda
½ teaspoon salt
1½ cups quick-cooking rolled oats
1 (14-ounce) can sweetened condensed milk
1 (12-ounce) package semisweet chocolate chips
¼ cup margarine
2 teaspoons vanilla

Preheat oven to 350 degrees F. Grease a 9 × 13-inch baking pan and set aside.
Cream together butter and brown sugar in a large mixing bowl; add eggs and vanilla. Sift flour, baking soda, and salt into a small bowl, and then add to creamed mixture. Mix in oats.
Combine sweetened condensed milk, chocolate chips, and margarine in a heavy saucepan, and heat until chocolate is just melted. Stir in vanilla.
Spread two-thirds of the dough into prepared baking pan. Spread with chocolate mixture. Drop remaining one-third of dough on top by spoonfuls. Bake 25 minutes. Cool and then cut into bars.