Preheat the oven to 300 degrees F. Beat the butter and icing sugar until light. Beat in zest and extract. Gradually beat in the flour and cornstarch until well combined. Mix in the pine nuts. Divide the dough in two. Flatten each half into a 1/2-inch thick disc, and then wrap and refrigerate for a hour. Roll out dough on a lightly floured surface to a 1/2-inch thickness. Use small - remember this is rich shortbread - cookie cutters to cut out the cookies. Place on non-stick or parchment-lined baking sheets. Bake 15-20 minutes, or until very pale golden in colour. Cool cookie trays on baking racks.
Note: To toast pine nuts, place in a single layer in a small baking dish. Bake, shaking the dish from time to time, in a preheated 300 degrees F oven for 10-15 minutes, or until lightly toasted. Cool to room temperature before using.