Preheat oven to 350 degrees. Line a 8x8-inch or 8x11 glass or metal baking pan with parchment paper, set aside. In the bowl of a stand mixer fitted with the paddle attachment or a large mixing bowl (using a hand held mixer), add the oil, sugars and vanilla. Mix on medium speed until combined, about 1 minute.
This next step is key! Add the eggs and yolk one at a time, mixing after each addition, then mix for 30 seconds. Let the mixture rest for 3 minutes, the repeat by mixing for 30 seconds. Repeat two more times.
In a medium mixing bowl whisk together the cocoa powder, flour, salt, and baking powder. Slowly add the dry ingredients and chocolate chips and continue mixing for 30 seconds until all the white streaks are gone.
Pour the batter into the prepared pan and place in the oven to bake for 25-35 minutes or until a toothpick inserted into the center comes out with a few moist crumbs. Remove from the oven and allow to cool in the pan for 30 minutes. Slicing, serve and enjoy!