In a medium bowl sift together flour, baking soda, and salt. Set aside.
In another medium bowl, mix together granulated sugar, brown sugar, and orange zest. Set aside.
In a mixer bowl fitted with the paddle attachment, beat butter and orange sugar mixture on medium speed for about 3 minutes, until light and fluffy. Beat in egg, then vanilla. Add flour mixture and mix as little as possible just until combined. Gently fold in chocolate. [Although optional, I highly recommend chilling the dough for several hours or overnight. This step results in thicker cookies that won’t spread while baking.]
Preheat oven to 350F/180C. Line 2 pans with parchment paper. If the cookie dough is hard to handle after refrigerating, leave it on the counter for a few minutes to soften.
Drop rounded balls of dough, the size of 3 tablespoons each (an ice cream scoop works great here) onto prepared baking sheet. Bake for about 10-12 minutes, until cookies just begin to brown at the edges but the center is still very soft. Cookies will appear underdone but will continue to bake on the warm baking sheet. Leave cookies on the pan for 10 minutes or until they are easy to move without breaking, then gently transfer them to a wire rack to cool completely.
Store cookies at room temperature in an airtight container for up to 3 days. They can also be frozen for up to 2 months.