Preheat the oven to 320°F (160°C).
Melt the butter, add sugar, and mix until combined (no need to whisk vigorously).
Add the egg, egg yolk, and vegetable oil, and mix again.
Stir in the cocoa powder until smooth.
In a separate bowl, combine the flour, cornstarch, baking powder, and salt, then mix into the chocolate mixture.
Fold in the dark chocolate chips and chopped hazelnuts.
Spread the batter into a 9x9-inch pan lined with parchment paper.
Bake for 30 minutes in the preheated oven.
Let the brownies cool on a wire rack before cutting and enjoying.