Step 1

Preheat the oven to 325° Fahrenheit. Grease an 8x8 baking dish, and set aside.

Step 2

In a large bowl, beat the melted butter and sugar until smooth. Add the eggs and vanilla, and whisk to combine. Stir in the oil and cocoa powder, mixing just until smooth.

Step 3

Using a rubber spatula, stir in the flour, cornstarch, baking soda, and salt just until combined and no clumps remain. Gently fold in the chocolate chips.

Step 4

Pour the batter into the prepared baking dish, spreading it out evenly.

Step 5

Bake 25-30 minutes or until the batter is set and a cake tester can be inserted into the center and removed with just a few moist crumbs attached. Be careful not to overcook!

Step 6

Set aside to cool.

Step 7

Store unsliced in an airtight container at room temperature for up to 4 days or sliced for 1-2 days.