Preheat oven to 300 degrees F. Beat butter and icing sugar until light. Beat in ginger and zest. Gradually beat in flour and cornstarch until all is combined. Divide dough in two. Flatten each half into a 1-inch thick disc. Wrap and refrigerate a 1/2 hour. Roll out dough pieces on a lightly floured surface until a 1/4-inch in thickness. Use Christmas-shaped cookie cutters to cut out cookies. Place on non-stick or parchment paper lined baking sheets. Bake 15-20 minutes, or until very pale golden in colour. Cool cookie trays on baking racks. Decorate cookies with icing and holiday candies if desired.