Mix flour, baking powder and salt. Set aside.
Beat butter, sugar, eggs and vanilla until light and fluffy. Gradually stir in flour mixture. Mix until smooth and well combined.
Form dough into a ball and wrap in saran wrap. Refrigerate for several hours or overnight.
Preheat the oven 375F degrees. Divide the dough into parts. Roll out one ball at a time with a floured rolling pin, 1/8 inch thick, on a lightly floured surface. Flour the cookie cutters and cut out different shapes.
Place the cookies on a cookie sheet 2 inches apart.
Bake 7 to 10 minutes. Cool the cookies in a rack before decorating.
FOR THE ROYAL ICING:: Beat the egg whites lightly, add the confectionary sugar and cream of tartar. The latter will make the icing hard so it gives that crunchy glaze on the surface. Mix everything together in spreadable consistency. Use different colors to add to the icing, place in different bowls, prepare the sprinkles, and anything else you want to decorate your sugar cookies. (If it gets too thick, add a little bit of milk to thin it out.)
