Preheat oven to 325 degrees. Whisk together flour, cocoa powder, baking soda, and salt in a medium bowl; set aside. Melt coarsely chopped chocolate with the butter in a small heatproof bowl set over a pan of simmering water.
Transfer chocolate mixture to the bowl of an electric mixer fitted with the paddle attachment. Add sugar, eggs, and vanilla; mix on medium speed until combined. Reduce speed to low; gradually mix in flour mixture. Fold in chocolate chunks.
Scoop batter using a 1 1/2-inch ice cream scoop; place 2 inches apart on parchment-lined baking sheets. Bake until cookies are flat and surfaces begin to crack, about 15 minutes. Transfer on parchment to wire racks. Let cool 5 minutes. Cookies can be stored in an airtight container at room temperature for up to 3 days.