For the Sugar Cookies:

1. 1. Preheat your oven to 350°F (175°C).
2. 2. In a bowl, whisk together flour and baking powder until combined.
3. 3. In another bowl, cream softened butter and sugar until light and fluffy (about three minutes).
4. 4. Add eggs one at a time, mixing well after each addition, then stir in vanilla extract.
5. 5. Gradually add the dry ingredients to the wet mixture until just combined; do not overmix.
6. 6. Roll out dough on a floured surface to about half an inch thick and cut into desired shapes.
7. 7. Place cookies on lined baking sheets and bake for 8–10 minutes or until edges are lightly golden. Allow to cool completely.

For the Royal Icing:

1. 1. While cookies cool, combine powdered sugar with egg whites (or rehydrated meringue powder) in a bowl.
2. 2. Mix until smooth but spreadable; adjust consistency if necessary by adding water or more powdered sugar.
3. Decorating:
4. 1. Once cookies are cool, use piping bags to outline and fill in each cookie with royal icing.
5. 2. Let them dry completely before stacking or serving.