Not only are the flavor of these cookies deepened by dark brown sugar but the entire cookie base gets a delicious start from browned butter. And if you need further evidence that browned butter is the secret to all magical desserts look no further than here and here.
Quite simple in concept, the cookies are unadorned with any specific add-ins, which allows the browned butter and brown sugar flavors to really shine through. Just a wonderfully delicious cookie and you can’t ask for much more than that, in my opinion.
Heat 10 tablespoons butter in a 10-inch stainless steel (not nonstick) skillet or small uncoated saucepan over medium heat until melted, about 2 minutes. Continue to cook, swirling pan constantly until butter is dark golden brown and has nutty aroma, 1 to 3 minutes. Browned butter can burn easily and quickly so keep a close eye on it! Remove the skillet or pot from the heat and transfer the browned butter to a large heatproof bowl. Stir the remaining 4 tablespoons butter into the hot butter to melt; set aside for 15 minutes.
Meanwhile, adjust an oven rack to the middle position and heat the oven to 350 degrees F. Line 2 large (18 by 12-inch) baking sheets with parchment paper, silpat liners or lightly coat them with cooking spray. In a shallow baking dish or pie plate, mix the granulated sugar and 1/4 cup packed brown sugar, rubbing between your fingers, until well combined; set aside. Whisk flour, baking soda, and baking powder together in a medium bowl; set aside.
Add remaining 1 3/4 cups brown sugar and salt to the bowl with the cooled butter and mix until no sugar lumps remain, about 30 seconds. Scrape down the sides of the bowl with a rubber spatula; add the egg, yolk, and vanilla and mix until fully incorporated, about 30 seconds. Scrape down the bowl. Add the flour mixture and mix until just combined, about 1 minute. Give the dough a final stir with a rubber spatula to ensure that no flour pockets remain and the ingredients are evenly distributed.
Divide the dough into 24 portions, each about 2 tablespoons, forming balls about 1 1/2 inches in diameter. Working in batches, toss the balls in the reserved sugar mixture to coat and set them on the prepared baking sheet, spacing them about 2 inches apart, 12 dough balls per sheet. (Smaller baking sheets can be used, but it will take 3 batches.)
Bake one sheet at a time until cookies are browned and still puffy and edges have begun to set but the centers are still soft (cookies will look raw between cracks and seem underdone - don't overbake), 11-13 minutes. Cool the cookies for 1-2 minutes on the baking sheets and then remove to a wire rack to cool completely.
Butter: if you plan to use unsalted butter, increase the salt in the recipe to 1/2 teaspoon.
Brown Sugar: I wouldn't recommend substituting light brown sugar - because the brown sugar makes up the majority of flavor in the cookies, the dark brown sugar adds depth and substance.
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