Preheat oven to 375 degree.
Spray and foil a 9 X 9 inch square pan.
Beat together butter, white sugar and brown sugar.
Beat in egg and vanilla.
Add flour and baking soda. Mix until well combined.
Fold in chocolate chips.
Spread ½ of the cookie batter in the prepared pan.
Pour the can of sweetened condensed milk over the batter.
Spread the remainder of the cookie batter over the sweetened condensed milk, you don't have to cover all of the sweetened condensed milk.
Sprinkle with remaining chocolate chips.
Bake for 30-35 minutes. The condensed milk will bubble up through the batter in some places. Bake until golden but still slightly gooey in the middle.