Use electric mixer fit with paddle attachment to beat butter, sugar, brown sugar, and zest of two oranges until lightened and fluffy, about 3 minutes.
Beat in egg until well combined.
In a small bowl, whisk together flour, cocoa powder, baking soda, and salt. Gradually beat into butter mixture until well combined. Stir in chocolate chips.
Chill dough for 30 to 60 minutes.
Preheat oven to 350 degrees. Line a cookie sheet with parchment paper or silicone baking mats.
Form dough into balls ~1 tablespoon in size. Place on prepared cookie sheet, about 2 inches apart.
Bake for 11-13 minutes, or until tops no longer look wet. Let cookies cool for about 5 minutes, then sprinkle zest of remaining orange on top of cookies.