Flourless chocolate fudge cookies - GLUTEN FREE!

Deep Dark Chocolate Cookies
Adapted From Epicurious

Nonstick vegetable oil spray
1 1/2 cups bittersweet chocolate chips (about 9 ounces), divided
3 large egg whites, room temperature
2 1/2 cups powdered sugar, divided (I used 2 cups)
1/2 cup unsweetened cocoa powder
1 tablespoon cornstarch
1/4 teaspoon salt