Preheat your oven to 375 degrees F. Grease a 9 by 13-inch jelly roll pan with softened butter or cooking spray.
Beat your butter in the bowl of a stand mixer until it is light and fluffy, about 2 minutes. Add the brown and granulated sugars a little bit at a time, pouring them in on the side of the bowl. While you're adding your sugars, stop the mixer occasionally and scrape the sides and bottom of the bowl to make sure that everything is fully mixed. Cream the butter and sugar mixture until it is light and fluffy, about 8 minutes. (Creaming creates air pockets in the dough, and therefore these blondies have a light and delicate texture.)
Whisk your flour, baking soda and sea salt, if using, in a bowl. (Add the salt only if desired because there is plenty on top of the finished brownies.) Add your eggs 1 at a time, adding the second only after the first is fully incorporated. Once the second egg is fully incorporated, add your vanilla. Turn your mixer down to low speed and slowly add the flour mixture. Once the dry ingredients are fully incorporated into the wet ingredients, add in all those chocolate chips. Your machine will seem to be struggling to mix in the chocolate chips because there are so many of them, but don't turn it up, leave it on low and allow it to fully mix the chips in. Spread your cookie dough into the prepared jelly roll. Evenly sprinkle the top of the dough with the fleur de sel.
Pop the blondies in the oven until they are golden brown on top and not wobbly in the center, 30 to 35 minutes. Allow the blondies to cool in the pan on a wire rack or on a dishtowel on your counter.
Cut the blondies, still in the pan, with a sharp knife into bars, about 48 pieces. Remove with a spatula and serve. (Store any leftovers in an airtight container for up to 1 week, but they will not last that long!)
Recipe courtesy of Anne Thornton