Preheat the oven to 325 degrees Fahrenheit. Grease an 8x8 baking dish, and set aside.
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.
8 Tablespoons unsalted butter, 1 cup granulated sugar, 2 large eggs, 1 teaspoon vanilla extract, 2 tbsp vegetable oil, 3/4 cup unsweetened cocoa powder
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.
1/2 cup all-purpose flour, 2 teaspoons cornstarch, 1/4 teaspoon baking soda, 1/2 teaspoon salt, 1 cup semi-sweet chocolate chips
Pour the batter into the prepared baking dish, spreading it out evenly.
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!
Set aside to cool.
Store unsliced in an airtight container at room temperature for up to 4 days or sliced for 1-2 days.