Preheat the oven to 350°F Grease and line an 8×8 baking pan with parchment paper or aluminum foil. Cut two strips the same width as the pan so that all sides of the pan are covered in parchment paper. (You can use a 9x9-inch pan but it will cook quicker.)
In a bowl, sift together the flour, cornstarch, cocoa powder, and salt.
Melt your butter, chocolate, and oil in a microwave-safe bowl in the microwave at 30-second increments. (Read HOW TO MELT CHOCOLATE)
Using a hand or stand mixer with the whisk attachment, whisk together the eggs and sugars on high until light and fluffy. This will only take a couple minutes.
Make sure the chocolate isn’t too hot. Turn the mixer down to low and slowly pour in the chocolate. Mix in the vanilla.
Fold in the dry ingredients using a rubber spatula. Be careful not to over mix.
Pour the batter into the prepared pan and smooth the top evenly. Bake for 20-22 minutes.
Let the brownies cool completely in the pan before removing and cutting.