First, heat the oven to 375 degrees F and line your baking sheets with parchment. Whisk the flour, baking soda and baking powder together and set aside, and stir the dredging sugars together in a separate small bowl.

Next, put 10 tablespoons of the butter in a light-colored heavy skillet over medium heat, swirling occasionally. It will foam and then quiet down as the water boils off. The moment the specks on the bottom turn hazelnut brown and it smells nutty, take it off the heat and stir in the remaining 2 tablespoons of cold butter.

Then pour the warm browned butter into a mixer bowl fitted with the paddle, add the dark brown sugar and salt, and beat until combined. Let it cool to warm rather than hot if the butter is still steaming.

After that, add the egg, the extra yolk and the vanilla and beat again, scraping down the sides and bottom of the bowl at least once.

Add the flour mixture in three additions on low speed, scraping between each, and stop the moment the flour disappears. The dough will be very thick. Fold in the butterscotch chips now if you are using them.

Roll tablespoon-sized pieces of dough into balls, keeping every ball in a batch the same size so they bake evenly, then dredge each one in the sugar mixture until well coated.

Finally, space them well apart on the sheets, sprinkle each with a few flakes of sea salt, and bake for 10 to 12 minutes, or 13 to 15 for large ones, until the edges have just browned and the centers still look underbaked. Cool on the sheet for one minute before moving them to a rack.