Sift the flour, corn starch, salt, and baking powder in a large bowl. Whisk to combine and set aside.
Beat the butter and sugar until it is light and fluffy, then add the eggs one at a time.
Stir the flour in, making sure to add in several batches. Once combined cover with plastic and chill in the refrigerator for at least two hours.
Roll out the cold dough in small batches, It warms up quickly! You can roll out between two sheets of plastic wrap if you're worried about the dough getting too sticky to release.
Cut into heart shapes with a cookie cutter. If you do not have any heart-shaped cookie cutters just cut one out of paper and trace around it with a pin or small knife.
Lay on a baking sheet lined with parchment paper. Bake at 375F for 5 minutes, rotate the tray and bake additional 3-6 minutes depending on the size, thickness and desired crisp.
Allow time to cool on a rack while you whip up some royal icing.
Sift the powdered sugar into a bowl.
Beat the egg whites and add the cream of tartar once they become frothy.
Add the sugar in 3-4 batches and mix at low speed.
Once combined, add the water until desired consistency is reached.
Stir in the food coloring.