Whisk the wet ingredients

• Whisk the melted butter with the sugar.
• Add the eggs, egg yolk and vanilla extract and whisk until smooth.
• Whisk in the oil.

Add the dry ingredients

• Add the cocoa powder, flour, baking soda, salt and cornstarch.
• Whisk until just combined.

Fold in the chocolate chips

• Add the chocolate chips and whisk one last time until evenly distributed.

Bake

• Pour the batter into a lined baking dish.
• Bake at 325 °F until the edges are set and the center is slightly fudgy.

Cool and cut

• Let the brownies cool completely in the dish before cutting into squares.