Preheat your oven to 350 degrees Fahrenheit and prepare 2 baking sheets by lining with parchment paper. Set aside.
In a medium bowl, sift the flour, cornstarch, cream of tartar, baking powder and salt together through a sifter or sieve at least twice. This will ensure ultra fluffy cookies.
Using a stand mixer or hand mixer, beat the butter on high speed until it's light and fluffy and very creamy.
Beat the sugar into the butter until the mixture is creamy.
Add the vanilla, egg and egg white and mix just until combined.
With the mixer running on low speed, add the flour mixture slowly, mixing as you go. Stop mixing when no streaks of flour remain (don't forget to scrape down the bowl to ensure everything is incorporated!).
Using a cookie scoop or your hands, scoop out approximately tablespoon-sized amounts of dough and roll them into balls, placing them spaced out on the prepared baking sheets.
Flatten the dough balls down with a flat-bottomed drinking glass or measuring cup.
Bake at 350 degrees Fahrenheit for about 9-10 minutes, or until the cookies just lose their shine on top.
Let the cookies cool on the pans until they're firm enough to move to cooling racks to cool completely.
While the cookies are cooling prepare the frosting by whipping the butter and powdered sugar together until smooth and creamy.
Beat in the vanilla and add the heavy cream one tablespoon at a time just until the frosting reaches the perfect thick-but-spreadable consistency.
Add some green or red gel food colouring, if desired, and mix until the desired colour is reached.
When the cookies have cooled, use a small offset spatula or the back of a butter knife to frost the cookies with a generous dollop of frosting.
Top them with sprinkles and serve!
These cookies keep well in an airtight container for about 2 days at room temperature. The frosting will transfer, so it's best to store them in a single layer or with parchment paper between them