Preheat oven to 350 F. Grease an 8-inch square baking dish. Set aside.
Melt butter in a large, heavy saucepan over low heat.
Stir in cocoa powder until well-combined.
Remove from heat, and stir in sugars and vanilla.
Add eggs, and stir in.
Stir in cornstarch and salt.
Fold in chocolate chips.
Pour into prepared baking dish and smooth top with a spatula.
Bake until edges are firm and center is just set, about 25-30 minutes. A toothpick inserted in the center should come out with fudgy crumbs attached. Do not overbake.
Let cool in pan to room temperature.
Place, uncovered, in freezer for at least 1 hour before cutting and serving.
Notes
Can sub regular chocolate chips.
If you need the brownies to be gluten-free, be sure to check the labels on your ingredients to ensure that they are gluten-free.