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. Do not overbake. Transfer on parchment to wire racks. These cookies will be soft when you take them out of the oven, but they will firm up.
Let cool 5 minutes. Cookies can be stored in an airtight container at room temperature for up to 3 days.