In the bowl of a KitchenAid mixer, add 1 cup Butter and 1 cup Sugar.
Cream the butter and sugar until soft.
Add 1 Egg, 1 1/2 tsp Vanilla Extract and mix until just combined.
Measure and then add 2 Tbsp Milk, then mix again.
To the butter mixture, measure and then add 3 cups Flour, 1/2 tsp Salt, 1/2 tsp Cream of Tartar and 1 1/2 tsp Baking Powder.
Add the paddle attachment, plug in the mixer and then mix on LOW until combined.
Use a spatula to scrape down the sides, if needed, then continue to blend the ingredients together.
Remove the paddle attachment and then remove the bowl from the mixer. Place the Sugar Cookie dough, covered, in the refrigerator and chill for 30 minutes.
Preheat the oven to 350 degrees Fahrenheit.
Remove the dough from the refrigerator.
Flour a clean surface and then turn out the Sugar Cookie dough onto the surface.
Use a rolling pin to roll out the dough to 1/4-inch thick.
Use a cookie cutter to cut out the shapes.
Place the cut-outs onto a Baking Stone, 2 inches apart. Sprinkle the Sugar Cookies with Rainbow Sprinkles.
Bake the cookies in the oven for 8-12 mintues, or until just slightly brown on the edges. DO NOT OVERBAKE!
Remove the Baking Stone from the oven.
Rest the Rainbow Sprinkle Sugar Cookies on the baking stone for 5 minutes.
Use a spatula to remove the cooled Sugar Cookies to a cooling rack to cool completely.