How to Make It
Preheat oven to 350°F. Combine bittersweet chocolate, unsweetened chocolate, and butter in a microwaveable bowl. Microwave on HIGH until chocolate melts, about 2 minutes, stirring every 20 seconds. Cool to room temperature, about 10 minutes.
Whisk together flour, baking powder, and salt in a small bowl.
Place sugar and eggs in a large bowl. Beat with an electric mixer fitted with the whisk attachment on medium-high speed until light and fluffy, 10 to 12 minutes. Beat in chocolate mixture and vanilla. Sprinkle flour mixture over sugar mixture and fold in just until combined. Fold in chocolate chunks.
Drop dough by scant 3 tablespoonfuls 4 inches apart on parchment-lined baking sheets. Bake in preheated oven until tops are cracked but still very soft, 13 to 15 minutes. Cool on baking sheets 5 minutes. Transfer cookies to wire racks to cool completely.