Preheat oven to 350°F (175°C).
In a large bowl, cream together butter, granulated sugar, and brown sugar until light and fluffy.
Beat in the egg and vanilla extract.
In another bowl, whisk together flour, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Fold in the dark chocolate chips and orange zest.
Divide the dough in half and shape each half into a log about 2 inches in diameter. Wrap in plastic wrap and refrigerate for at least 1 hour.
Slice the dough into ¼-inch thick rounds and place on a baking sheet lined with parchment paper.
Bake for 10-12 minutes, or until edges are lightly golden. Cool on a wire rack.