Cream the butter and sugar together until fluffy.

Mix in the vanilla extract.

Beat in the condensed milk and the flour, alternately, mixing after each addition.

Mix in the chocolate chips

Spray a 8x8 square baking pan with cooking spray and line it with parchment paper with long ends so you can lift out the bars for easy cutting later. You can skip the paper if you want to cut and serve them right out of the pan.

Turn the dough out into the pan and pat down evenly with your hands. You may want to flour your hands as the dough is sticky. Smooth it out evenly

Cover and refrigerate until firm, (about 4 hours) or even overnight if you like. The texture will be similar to a fudge.

To make the chocolate topping, put the semi sweet chocolate chips in a microwave safe bowl or measuring cup and microwave for one minute. Stir, and then return to the microwave for short bursts of 15 seconds until the chocolate is totally smooth and melted. 

Let the chocolate cool for about 5 minutes, and then pour onto the dough. Spread out evenly. Return to the refrigerator until the topping is firm.

Cut into 16 squares. Keep the bars in the refrigerator until ready to eat.