Preheat oven to 325 degrees F. Whisk together flour, cocoa powder, baking soda, and salt in a medium bowl.
Melt the 4 ounces coarsely chopped chocolate with the butter in a small microwave-safe bowl in 15-second bursts on high until melted and smooth. Let cool slightly.
Put the melted chocolate mixture, sugar, eggs, and vanilla in the bowl of an electric mixer fitted with the paddle attachment. Mix on medium speed until combined. Reduce speed to low; gradually mix in flour mixture. Fold in chocolate chunks.
Using a 1 1/2-inch spring-loaded cookie scoop, drop dough onto parchment paper-lined baking sheets, spacing 2 inches apart. Bake until cookies are flat and surfaces crack, about 12 minutes. Let cool on parchment on wire racks. Cookies can be stored in an airtight container at room temperature for up to 3 days.