Preheat oven to 350 degrees F. Line two 8-inch cake pans with rounds of parchment and grease the sides with cooking spray or flour and butter. Set aside.
Using a vegetable peeler, shave the chocolate. If they’re long shavings, transfer to the fridge to chill and then break up with your hands.
In a medium bowl, whisk together the all-purpose flour, cornstarch, baking powder, baking soda and salt. Set aside.
In a large measuring cup, measure out the milk and whisk in the sour cream until mostly smooth; add the egg whites. Whisk together and set aside.
In the bowl of a stand-up mixer (or using a large bowl and an electric hand mixer), add the butter, sugar, neutral oil and vanilla extract or vanilla paste. Beat until fluffy, about 2 to 3 minutes. Next, rotating between the milk/egg white mixture and the dry ingredients to the bowl, ending and beginning with the liquid mixture. Beat for an additional 20 seconds, until everything is combined and cohesive. Give it a manual stir with a spatula just to ensure all of the flour at the bottom of the bowl is properly combined. Add the chocolate fold it in until combined.
Divide the batter amongst the two cake pans and using a spatula, even out the top and spread the batter to the edges of the cake pans. Transfer to the oven to bake for about 30 to 35 minutes, until a skewer inserted into the center comes out clean.
Remove from the oven and allow to cool for about 5 to 10 minutes in the pan before inverting them onto cooling racks.