In a mixing bowl, cream the butter and sugar together until smooth and not grainy. Add all the eggs at once and continue mixing until fully incorporated. Stir in the vanilla and mix until combined.

Sift the flour, baking powder and salt together, then add to the wet mixture. Continue mixing on low speed until all ingredients are thoroughly blended. Shape the dough into a ball, then wrap it completely in plastic wrap. Refrigerate the dough for about 6 hours.

Preheat the oven to 325 degrees. Remove the dough from the refrigerator and let it soften slightly so it can be rolled. Lightly flour your work surface and rolling pin, and incorporate a little flour into the dough if needed. Roll the dough out and use a cookie cutter to cut your cookies. Line a cookie sheet with parchment paper and carefully transfer the cut cookies onto the sheet using a metal spatula. Bake at 325 degrees for 8 minutes. Once baked, remove the cookies from the sheet and place them on a wire rack to cool completely.

To make the royal icing, combine the egg whites, powdered sugar and cream of tartar in a mixing bowl. Beat on medium speed for about 4 minutes. If the icing is not forming stiff peaks, add an additional ¼ cup of powdered sugar. Use the icing to decorate the cookies. There are many tutorials available online for decorating with royal icing — remember, practice makes perfect!