Preheat the oven to 350 degrees F.
Combine the flour, cocoa powder, baking soda and salt in a large bowl. Mix the butter and sugars until combined. Add eggs, syrup and vanilla and beat to combine. Stir in the dry ingredients in batches until combined. Using a wooden spoon, stir the chocolate chunks and chips into the dough until well dispersed. Chill 30 minutes or overnight.
Drop by rounded tablespoons (1 1/2-inch balls) onto parchment-lined sheet trays about 3 inches apart. Bake until lightly browned, 16 to 18 minutes. Cool on tray 3 minutes. Transfer to cooling racks and cool as long as you can resist.