In a bowl, sift together cornstarch, powdered sugar & flour. Blend in butter & flavoring with a spoon, mixing until a soft, smooth dough forms. If dough is too soft to handle, cover & chill about 1 hour.
Between 2 sheets of parchment paper, roll dough out into a rectangle about 12″ x 7″ & 1/2-inch thickness. Make 7 strips on the longest side & 6 strips on the short side. Transfer to ungreased baking sheets spacing 1 1/2-inches apart. Place baking sheets in refrigerator & chill 30 minutes. Halfway through preheat oven to 300 F.
Bake for about 20 minutes or until edges are just barely browned.
When shortbread is cooled, decorate with icing & sprinkles to create Christmas tree design.
Recipe Notes
The almond flavor is optional but I think it adds a nice touch.