Preheat the oven to 350°F (175°C).

In a large bowl, cream together the unsalted butter and confectioners sugar until light and fluffy.

Mix in the vanilla extract, orange zest, and chopped rosemary until well combined.

Gradually add the all-purpose flour, cornstarch, and salt to the butter mixture, mixing until just combined.

Fold in the dark chocolate pieces and candied orange peel.

Roll the dough into logs and wrap in plastic wrap. Chill for 30 minutes.

Slice the chilled logs into 1/4-inch thick rounds and place on a baking sheet.

Bake for 15 minutes or until the edges are lightly golden.

Allow to cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.