Preheat oven to 350°F. Spray bottom and sides of 9X13 inch pan with cooking spray.

With an electric mixer with a paddle attachment, cream 1 cup butter with both sugars and the eggs.

In a large bowl, mix the flour, baking soda, salt and oats and add slowly to the creamed mixture.

Spread three-quarters of the batter into the prepared pan, pressing down dough evenly with wet hands. Reserve the remainder of dough for topping.

Combine condensed milk, chocolate chips, the remaining 1/2 cup of butter and vanilla in a microwave safe bowl; melt in the microwave, stirring often to prevent scorching. Stir until smooth and pour chocolate mixture over dough in pan.

Drop the remainder of oat mixture by spoonfuls on top - press down slightly.

Bake for 25-30 minutes, or until top is lightly browned (chocolate mixture will still be moist but will set or harden as it cools down). Let cool completely before cutting and serving.