- Nonstick vegetable oil spray
- 1 cup bittersweet chocolate chips (do not exceed 61% cacao; about 6 ounces), divided
- 1 1/2 cups powdered sugar, divided
- 1/3 cup unsweetened cocoa powder
- 2 teaspoons cornstarch
- 2 large egg whites, room temperature
- 1/2 teaspoon vanilla extract
- 1/8 teaspoon cream of tartar
- Position 1 rack in top third and 1 rack in bottom third of oven and preheat to 350°F. Coat 2 large rimmed baking sheets with nonstick spray. Place 1/2 cup chocolate chips in small microwave-safe bowl. Cook in 15-second intervals until chocolate softens; stir until melted and smooth. Cool chocolate to lukewarm, about 10 minutes. Whisk 1/2 cup sugar, cocoa, and cornstarch in small bowl to blend.
- Using electric mixer, beat room-temperature egg whites, vanilla, 1/8 teaspoon salt, and cream of tartar in medium bowl until soft peaks form. Add remaining 1 cup sugar in 4 additions, beating just to blend after each addition. Continue to beat until meringue is thick and glossy like marshmallow creme, about 2 minutes longer. Beat in cocoa mixture. Fold in melted chocolate, then 1/2 cup chips.
- Drop batter by rounded tablespoonfuls onto prepared sheets, spacing 3 inches apart. Bake cookies 7 minutes. Reverse sheets and bake until dry-looking and cracked, about 6 minutes. Cool cookies on sheets 5 minutes. Transfer cookies to racks and cool completely.