In a large bowl, whisk together sugar, butter, eggs, vanilla, and vegetable oil. Whisk in cocoa powder, flour, and cornstarch until smooth and well combined. Fold in chocolate chips.
Spray a 6" round baking pan with nonstick cooking spray. Transfer batter to prepared pan and cover tightly with aluminum foil. Pour water into an Instant Pot® and place the trivet in the pot, then place pan on trivet. Place the lid on the Instant Pot® and click into place to close.
Press the Pressure Cook button and adjust the timer to 40 Minutes. When the timer beeps, let the pressure release naturally for 10 Minutes then quick-release any remaining pressure until float valve drops. Unlock lid. When done, brownie will be firm to the touch and a toothpick inserted into the center will come out clean. Serve warm.