Sift cornstarch, icing sugar, flour and salt into a bowl.
Matcha Shortbread Cookies :Add the 2.5 tablespoons matcha green tea now and sift into the mixture. Do not add matcha if you want to make vanilla shortbread cookies.
Vanilla Shortbread Cookies: Add the 1 teaspoon of vanilla after you have completed the sifting. Do not add if you want to make matcha shortbread cookies.
Place either your matcha green tea or vanilla flour mixture into a food processor and add the softened butter. You do not want you butter melted but just at room temperature, if you forget to soften your butter you can put it in the microwave at 50 percent power for 10 second intervals until softened and you can put a dent easily with your finger). Optional: If you want to add sesame seeds directly to vanilla shortbread cookie dough, you can add one tablespoon of either black or white sesame seeds at this time. Pulse together gently until a dough ball forms. Alternatively you can process your dough with your hands or a spoon.
Shape your dough into a round disk if planning on making cut out cookies and wrap tightly with plastic wrap. Do not overwork the dough or it will make your cookies tough. The less touching the dough the better. If you want to just cut slices out of your dough form your dough into a log about 6 inches long and 2 inches wide and wrap tightly and place in the refrigerator for at least 2 hours or up to overnight. (Refrigeration is a very important step)
Preheat your oven to 300 degrees F or 150 degrees C. Take your dough out of the refrigerator and remove plastic wrap.
a) If you are planning on just slicing the cookies from the log then cut into about ¼ inch thick slices and place on a greased baking sheet or one lined with waxed/parchment paper about a couple of inches apart. Your cookies should not expand. Really you can shape the dough in any form you like. My mom used to press the dough into a plastic lined baking dish and refrigerate and then cut them into rectangles and then decorate with fork holes before baking.
b) If you are planning on making cutout shortbread cookies with sakura flower cookie cutters, remove the plastic wrap from the circle dough and allow to soften slightly before trying to roll out the dough.
Generously flour your work service. Roll out your dough so that dough is about ¼ inch thick. The little bit thicker dough results in a lovely cookie. Flour your cookie cutters and firmly press into dough and repeat the process for the rest of the dough. I used a spatula to move the cookies from my counter to the parchment line or greased pan. Repeat the process for the rest of the dough. Sometimes the dough will be soft so just wrap it back up into a ball and cover with plastic wrap and put in the refrigerator for a few minutes until it firms up again, roll flat and repeat the process.
Decorate the cookies with sesame seeds or sugar confetti as desired. Press the sesame seeds down into the dough so they stick. If you want to ornately decorate the side of the cookies with sprinkles then first brush on a little egg white and then dip into sprinkles. (For an extra layer of security to make sure that my cookies did not expand I put the cookies on the cookies tray in the refrigerator for just a few minutes before baking. You probably can skip this step.)
Bake sakura shortbread cookies for 15-20 minutes or until the edges are slightly tan.
Cool on a wire rack.
Decorate cookies with royal icing, sprinkles, and decorations as desired.
Enjoy Sakura Shortbread cookies with a nice hot cup of matcha, tea or coffee. Store your cookies in a plastic sealed container and if for some reason your family does not eat them within 3 days put them in the freezer and they can be stored up to 3 months.