Preheat oven to 350 degrees and line a 9x9 baking dish with non-stick foil (or spray with baking spray).
In a bowl, cream butter, brown sugar, and granulated sugar together (you can do this with a hand mixer, standing mixer, or by hand with a rubber spatula). Once well combined, add in the egg, and mix until well combined.
Add in the baking soda, salt, espresso powder, and vanilla extract. Mix until combined.
Add in flour and cornstarch, mixing until combined.
Lastly, add in butterscotch chips and chocolate chips.
Press dough into your prepared baking dish (making sure the dough is spread in an even layer).
Bake for 20-22 minutes, or until the bars are lightly golden brown and set.
Allow to cool before slicing.
Enjoy!