In a medium-sized bowl, whisk together the flour, baking soda, cornstarch and salt. Set aside.
In a large bowl, mix together the butter with both sugars until creamy.
Add egg and then add vanilla extract. Mix until just combined.
With the mixer on low incorporate the flour mixture.
Fold in the chocolate chips or chunks.
Spread the dough on top of the brownie batter.
Bake for about 50-60 minutes until a toothpick inserted in the center comes out with a few moist crumbs sticking to it.
While baking, cover with aluminum foil if the top is browning too quickly.
Let it cool at least 30 minutes before serving.
Enjoy slightly warm with a good dollop of ice cream and melted chocolate.
Keep leftovers covered at room temperature for up to 5 days.