Instructions

• Preheat the oven to 350°F. Have a 9x13-inch baking dish ready.
• In a small bowl whisk together 2 cups all-purpose flour, ½ teaspoon baking soda, and ½ teaspoon salt.
• Place 1 cup unsalted butter in a skillet over medium heat. Cook, stirring occasionally, until the butter foams and the milk solids turn golden brown and brown bits form on the bottom. Once brown bits appear, turn off the heat and continue stirring about 30 seconds. Remove from heat and let the brown butter cool until it is warm but not hot.
• Use a pastry brush or a piece of parchment to spread a little of the cooled brown butter over the bottom and sides of the 9x13-inch dish to grease it.
• Put 1 1/2 cups light brown sugar in a large bowl. Pour the remaining warm brown butter into the sugar and mix until combined and smooth—the sugar will partially dissolve.
• Add the 2 lightly beaten eggs one at a time, stirring after each addition so the eggs do not cook if the mixture is still warm. Stir in 1 tablespoon vanilla extract.
• Use a wooden spoon to stir in the dry ingredients from step 2 until they are almost combined. Do not overmix.
• Stir in 1 cup milk chocolate chips and 1 cup dark chocolate chips until evenly dispersed.
• Press the mixture evenly into the prepared 9x13-inch baking dish.
• Bake for 25–30 minutes, or until the top is golden brown and the center is set (a toothpick inserted in the center should come out with a few moist crumbs, not wet batter).
• Remove the pan from the oven and immediately sprinkle the top with Maldon flaked salt. Let the bars cool completely in the pan before slicing into squares.