Preheat the oven to 350 degrees F. Line a baking sheet with a silicone mat.**
In a large heatproof bowl, melt the butter in the microwave until it's just barely melted, watching closely that it doesn't come to a boil or get too hot. If the butter becomes overheated, make sure you let it cool down before proceeding.
Add the brown sugar and granulated sugar to the bowl, and mix to combine, either with a hand mixer or by hand with a whisk. Only mix it just enough so that it's relatively smooth.
Add the vanilla extract and egg, and mix again until just combined.
In a separate bowl, whisk to combine the flour, baking soda, and salt, then add this to the wet ingredients. Stir until the flour has almost disappeared, then add the chocolate chips, and continue stirring until the dry ingredients have been absorbed.
Use a medium scoop (#40/1.5T) to portion the cookie dough onto the prepared baking sheet. You should get around 18 cookies, so you can either bake two batches one after another (my preference) or add another baking tray.
Sprinkle the tops of the cookies with fleur de sel, if using, then bake for 11-12 minutes, until the cookies look slightly golden on the edges, and soft in the middle, but with no visible glistening rawness in the center.
Remove the cookies from the oven, and allow them to cool for a few minutes, during which time they'll flatten a bit and cook a little more. Then enjoy while warm!
*No need to remember to pull the egg out in advance. Just put the egg in a bowl of warm water for a few minutes, and it's ready to go. The reason we warm the egg slightly is because otherwise a cold egg from the fridge will re-solidify some of the melted butter, and we want to keep everything smooth.**I have tested this recipe on several baking surfaces, and I find that the cookies brown the best on the bottom and spread the least when baked on a silicone mat, like a Silpat. Parchment paper is the second best option, followed by directly baking on the sheet pan. The cookies will turn out fine on any baking surface, just know there will be slight variations.