For the cookie dough: Add the flour, baking soda and salt to a large bowl. Using a whisk, stir the mixture to incorporate all the ingredients.
Cream the butter in a stand mixer until it lightens up. Slowly add in the sugars and continue to cream until the mixture is light and fluffy. Add the eggs 1 at a time, beating well after each egg. Stir in the vanilla. On low speed, add the flour mixture. Once incorporated, fold in the chocolate chips by hand.
Take 2 teaspoons dough, or if using a spoon, take a good scoop and roll it into balls. I use my hands, it's faster and a whole lot easier. Put the cookie dough balls onto a baking sheet and pop them into the freezer until firm, about 30 minutes. Chilling the dough will help the batter stick to the cookie balls.
For the batter: Combine the flour, sugar and baking powder in a large bowl. Take a whisk and give the mixture a few swirls to incorporate the ingredients and break up any lumps. Whisk in the egg and the seltzer, making sure to combine well. You want the batter to have the consistency of heavy cream, so add more seltzer, if needed, to achieve this.
In a large, heavy-bottomed stockpot, heat the oil to 350 degrees F.
Working in batches, add 3 chilled dough balls to the batter and turn to coat completely. Using a fork or slotted spoon, remove each cookie dough ball from the batter, letting the excess batter drip back into the bowl, then very carefully slip it into the hot oil and repeat.
Fry no more than 3 at a time, since you do not want the temperature of the oil to drop. Using a slotted spoon, turn them over a few times until they are golden brown, about 3 minutes total. Remove the crispy cookie dough balls from the oil to a paper towel-lined plate to drain. Sprinkle with a generous pinch of fleur de sel and serve warm. Enjoy!