Preheat oven to 350°F. Line sheet pans with parchment.
Cream butter and both sugars until lightened.
Add egg and vanilla; mix until smooth.
Add flour, cocoa, baking soda, and salt. Stir until just combined.
Fold in chocolate chunks, reserving some for topping.
Scoop into 1 1/2-tablespoon portions, 2 inches apart. Press extra chunks on top.
Bake 10–12 minutes. Cool on pan 5 minutes before transferring to a rack.
For a stronger chocolate punch, use a mix of bittersweet and milk chunks. These double chunk chocolate cookies also freeze well for up to 2 months.