Preheat the oven to 350°F(177C°). Grease 8 inch (20cm) square oven-proof dish and line the bottom and sides with parchment paper, about 2 inches(5 cm) of overhang on each side.
Mix flour, rolled oats, cornstarch, salt (if you are using) and baking soda in a medium bowl. Set aside.
Melt the butter halfway in a small saucepan over medium heat. Remove from the heat and stir until it is all melted.
Place the sugar in a large bowl and add the melted butter, stir. Then add the egg and stir, mix in the vanilla.
Add the flour mixture in 3 batches. Use a spatula after the 2nd batch as it will be easier to mix. When combined well, add in the chocolate chips and stir with your spatula.
Place the dough into the dish. Bake for 20-25 minutes or until the top sets. When you insert a toothpick in the middle, it should come out clean with little crumbs but it shouldn't be wet. Make sure not to overbake so the bars can stay soft and not dry. If you like, you can add extra chocolate chips while it is still hot.
When it totally cools, cut into 16 pieces with a sharp knife.