First, line your 20 cm seamless square baking dish with parchment paper. Line it tall so that when you pour your batter it won't spill out.
Secondly into a mixing bowl, sift your all purpose flour, cornstarch, cocoa powder and hot chocolate powder. It is really important to remove any clumps or large granules. Whisk until well combined.
Then separate your egg yolks from the whites into two bowls.
Microwave your oil for 80 seconds until it's 90 degrees C hot. Quickly pour the hot oil into the flour mixture and whisk well.
Whisk in about 20 grams of your chocolate chips only, reserving the remaining to top your cake with. Allow the chips to completely melt.
Once they have melted, whisk in half of your oat milk.
Then whisk in your egg yolks until completely mixed
Whisk in the remaining amount of oat milk. Set aside.
To your egg whites, add ⅓ of the white sugar. We will be adding the sugar in three portions - this is also important. Beat egg whites until you reach a soft peak, not a stiff peak.
Note: When you stir the egg whites with your whisk and lift it up, it should create a wiggly tail. If the meringue is too stiff, this will cause cracks in your cake.
Preheat your oven to 300 degrees F.
To your chocolate batter, mix in ⅓ of the meringue to loosen it. Whisk gently in order not to lose the fluffiness. Add the meringue in three portions. Using a spatula at the end to fold the mixture scraping the bottom of the bowl to ensure everything is properly mixed.
Pour your cake batter into the lined 20 cm square baking dish. Even out the top with a spatula. To break up the air bubbles, gently tap the square pan on the table a few times.
Then in your larger baking dish, pour hot water that is 70 degrees C warm into the tray, just enough to fill the tray about ⅓ of the way enough to make your square pan float. Place the square dish with your cake batter into this large pan.
Place into the oven to bake for 10 minutes.
At the ten minute mark, remove from oven to add in your remaining chocolate chips. If you add it in early, the chips will sink to the bottom.
Bake for 75 minutes until the top of the cake is solid but bounces back if you touch it. Poke a toothpick to check if the cake is fully cooked.
Once cooked, remove the cake from the pan. Peel away the parchment paper while hot.
Slice while hot and enjoy warm or at room temperature.