Place 1 lb of softened butter into the bowl of a stand mixer. Cream for 30 seconds, then add vanilla sugar and cream for at least 2 minutes more or until light in color and fluffy.
Blend in egg and 1/4 tsp salt, then slowly incorporate flour, beating until JUST combined. Do not overmix or your shortbread will be tough! Pat out into an 11x7 bar pan (I like my medium stoneware bar pan from Pampered Chef for this) and score into bars with a knife.
Prick with a fork (also called "docking") and place in preheated oven. Bake at 350F for 15 min then reduce temperature to 200F. and continue to bake for 1 hr.
Meanwhile, begin preparing icing.
Do not be a sillyhead like me and rush getting them out of the pan...or you'll end up with lots of broken pieces :)
Allow to cool completely. Once cool, place a small cookie sheet or sheet pan under cookies on cooling rack to catch excess icing. Using the tines of a fork, drizzle red then green icing over cookies. Allow to dry before storing in a sealed container.