Preheat the oven to 175 C / 350 F (no fan) and prepare a 9-inch / 23 cm brownie tin. Brush the inside of the pan with a tiny bit of oil, and then line the bottom and sides with parchment paper.
With an electric hand mixer, cream the room temperature butter with the granulated and brown sugar for a few minutes until light and fluffy.
Mix in the eggs one by one and then the vanilla, just until combined.
Mix the flour, corn starch, salt, and baking soda together, then add the mixture to the wet ingredients. Fold them together with the help of a rubber spatula.
Lastly, fold in the chocolate chips. Leave some for later.
Place the cookie bar dough into the prepared baking pan and smooth the top with an offset spatula. Spread the remaining chocolate chips on top.
Bake it at 175 C / 350 F for about 30 minutes. The bar is ready when the edge looks baked and the middle looks slightly gooey but not raw. Do not over-bake! The cookie bars will set further as it cools.
Let it completely cool in the pan before slicing and serving. Any leftovers can be stored in an airtight container at room temperature for 2-3 days.
Notes
INGREDIENT NOTES:
Measure your ingredients with a digital scale for accuracy.
Use room temperature ingredients, e.g. egg and butter.
TECHNIQUE NOTES:
Make sure you read my Expert Tips section above to maximize your success. A short recipe alone is not able to cover all the necessary details, and science behind baking.
Do not overmix the dough when adding the dry ingredients. Only fold it until thoroughly combined.
Try not to overbake your cookie bars if you prefer them fudgy and gooey.