Beat the butter, granulated sugar and brown sugar in a bowl with a mixer on medium speed until fluffy. Add the egg and vanilla; beat.
Whisk the flour, cocoa powder, baking soda and salt in a bowl. Add to the butter mixture and beat until just combined. Stir in the chocolate chunks.
Preheat the oven to 350 degrees F. Scoop rounded tablespoons of dough and roll into balls. Arrange on parchment-lined baking sheets about 2 inches apart. Refrigerate 30 minutes, then bake until the cookies are golden on the bottom, 12 to 15 minutes. Cool on the baking sheets, 10 minutes, then transfer to racks to finish cooling.