Cream the 1/2 cup unsalted butter and 3/4 cup packed light brown sugar and 1/4 tsp salt together until fluffy. I use electric beaters. Just beat it really good so it's light and creamy. Scrape down the sides of the bowl as necessary. Mix in the 1 tsp vanilla extract.
Blend in the 14 ounce sweetened condensed milk and then the 2 cups heat treated flour*.
Fold in the 10 ounces mini chocolate chips.
Spray a 9x9 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, I used an offset spatula to finish the job.
Cover and refrigerate until firm, (about 2-3 hours.) The texture will be similar to a fudge.
To make the chocolate topping, put the 1 cup semi sweet chocolate chips and 1 Tbsp neutral oil in a microwave safe bowl or measuring cup and microwave for 45 seconds. Stir until all the chocolate is completely melted.
Let the chocolate cool for about 5 minutes, and then pour onto the dough. Spread out evenly. Allow the chocolate to set up before slicing. Cut into 16 squares.
Keep the bars in the refrigerator until ready to eat.
Notes
*Note: regular flour is not safe to eat raw. See below for heat treated flour buying guide.