Preheat oven to 325°. Line baking sheets with parchment paper or silicone.
Whisk flour, cornstarch and salt (is using) in a bowl.
Beat butter and sugar in a separate bowl. Beat in the milk and vanilla.
Beat in flour mixture 1 tablespoon at a time. If using food colouring, add in a tiny, tiny bit and mix in evenly. Add more until you get the desired shade.
Attach piping nozzle of your choice to your pastry bag. Turn a couple of inches of the top cuff down and stand the bag up, nozzle down, in a deep cup. Add half the dough to the pastry bag. Tightly squeeze the top of the bag shut.
Pipe batter in 1 ½ " circles about 1" apart on the baking sheet.
Bake cookies 15 minutes. Check - edges should not be brown but may have just a hint of colour.While first sheet is baking, pipe the remaining batter on the 2nd baking sheet. Allow cooked cookies to cool about 5 minutes before transferring to a rack to finish cooling. Handle gently as cookies will be fragile until fully cooled.
If desired, seive icing sugar over completely cooled cookies.