Nonstick vegetable oil spray
2 cups (250 g) all-purpose flour
2 tsp. Diamond Crystal or 1 tsp. Morton kosher salt
¾ tsp. baking powder
¾ tsp. baking soda
¾ cup (1½ sticks) unsalted butter, melted, slightly cooled
1 cup (packed; 200 g) dark brown sugar
½ cup (100 g) granulated sugar
1 Tbsp. vanilla extract
2 large eggs, room temperature
2 cups semisweet chocolate chips (12 oz.), divided
Place a rack in middle of oven and preheat to 325°. Lightly coat a metal or glass 13x9" baking pan with nonstick vegetable oil spray, then line with parchment paper leaving a generous overhang on long sides. Whisk together 2 cups (250 g) all-purpose flour, 2 tsp. Diamond Crystal or 1 tsp. Morton kosher salt, ¾ tsp. baking powder, and ¾ tsp. baking soda in a medium bowl; set aside.
Whisk ¾ cup (1½ sticks) unsalted butter, melted, slightly cooled, 1 cup (200 g) packed dark brown sugar, ½ cup (100 g) granulated sugar, and 1 Tbsp. vanilla extract in a large bowl until combined and sugar is starting to dissolve. Add 2 large eggs, room temperature, one at a time, whisking until incorporated after each addition. Add reserved dry ingredients and stir with a rubber spatula until a soft dough forms and no pockets of unincorporated flour remain. Add 1⅔ cups semisweet chocolate chips (10 oz.) and stir just until distributed.
Transfer dough to prepared baking pan and press into an even layer with your hands or spatula coated with nonstick spray. Sprinkle remaining ⅓ cup semisweet chocolate chips (2 oz.) evenly over top of dough and press them in slightly to adhere.
Bake bar until top is golden brown and a tester inserted into the center comes out clean apart from any smudges of melted chocolate, 30–35 minutes. Let cool in pan at least 1 hour before cutting into an 8x3 grid to make 24 rectangles.