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
Step 1
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.
Step 2
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.
Step 3
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.
Step 4
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.