Preheat the oven to 375 degrees. Spray a 9-inch square cake pan with cooking spray. Line pan with parchment paper and lightly spray the parchment paper.
In a medium mixing bowl, combine the butter, flour, ⅓ cup sugar, and orange zest. Work into a dough using your fingers. Stir in ½ cup chocolate chips. Pat the mixture evenly and firmly into the prepared pan. Prick it all over with a fork. Sprinkle the remaining ⅓ cup sugar on top. Bake for 20-25 minutes or until lightly golden brown. Let cool completely in the pan. The cookies will firm up as they cool.
Remove the parchment and baked cookies from the pan when cooled. Cut in half, then cut each half into 6 finger sized rectangles.
Melt the remaining ¼ cup chocolate chips in a heat-proof pan over barely simmering water (making sure the bottom of the bowl does not touch the water) then drizzle the chocolate over the top of the cookies. Let sit for 10 minutes for the chocolate to firm up.