In a large bowl, beat softened butter with a hand whisk. Add brown sugar, grapeseed oil, and mix with the whisk. Then add half egg white, egg yolk, and, in order, flour, baking soda, and salt. Mix with a rubber spatula as little as possible. Finally, add dark and milk chocolate chunks. Cover the bowl with plastic film and refrigerate for 3 hours.
Make balls of 1.9 oz (55 g), place them on a baking sheet lined with parchment paper, cover with food film and refrigerate overnight.
The next day, heat oven to 375 F/190 C. Distribute cookie dough balls 3-inches/8 cm apart and bake for 9 minutes.
Let the cookies rest on the baking sheet for about 10 minutes before removing to a wire rack to cool completely.
