1. Preheat oven to 325 degrees. Whisk together flour, cocoa powder, baking soda, and salt in a bowl.
2. Melt 1 cup chocolate chunks with the butter (can easily do this in the microwave). Let cool slightly.
3. Mix chocolate mixture, sugar, eggs, and vanilla on medium until combined. Reduce mixer to low and gradually stir in flour mixture till just combined. Fold in remaining chocolate chunks.
4. Drop large balls of dough onto a parchment lined baking sheet, spacing 2 inches a part. Bake until cookies are flat and surfaces crack, about 15 minutes. Let cool on wire racks.