Preheat the oven to 350 F. Line an 8 x 8 baking pan with parchment paper, leaving extra around the edges to create overhang so you can easily lift out the brownies.
Add chocolate, butter, and both sugars together in a saucepan. Heat on medium. Mix until the chocolate and butter is halfway melted.
Remove from heat and stir until the butter and chocolate are completely melted. Allow the mixture to cool slightly, then vigorously mix in the oil, eggs and vanilla until the mixture is well-combined. About 3 - 4 minutes.I recommend using a whisk or even a hand mixer to whip the eggs into the mixture.
Mix in the flour, cornstarch, salt, and cocoa powder into the bowl until completely combined and there are no streaks of flour.
Pour the batter into the baking pan and bake for about 35 - 40 minutes, or until a toothpick comes out with a few thick, moist crumbs sticking to it.
Cool at least 10 - 20 minutes before attempting to remove the brownies from the an using the parchment. Cool completely before cutting.
Notes
Do not cut the brownies until they've had time to cool completely. Otherwise, it will be very difficult to cut and handle your brownies. Store them in an airtight container. If you'd like to try edible brownie batter, try my recipe!