Add the room temperature butter to a mixing bowl or bowl of a stand mixer. Cream together with the sugar until well combined.
Add in the egg and vanilla and beat together.
Add in the flour, baking powder and salt and beat together until a dough forms. Switch to a rubber spatula as needed.
Wrap the dough in plastic wrap and place in the fridge to set for at least 2 hours. Don't skip this!
After 2 hours, preheat the oven to 350 Fahrenheit.
Roll the dough to about 1/4 inch thick and use cookie cutters to cut into desired shapes. If the dough is a bit too firm, just let it sit for a few minutes.
I suggest rolling the dough on a floured piece of parchment paper to ensure nothing sticks.
Place onto baking sheets lined with parchment paper.
Bake for 10-13 minutes or until the cookies look set. If you like them chewy, you don't want to wait until the edges are too golden.
How long they need to cook really depends on the cookie cutters you use and how big they are, so keep an eye on them!
Let cool completely, then prep the icing by beating the egg whites for 1 full minute, then slowly add in the powdered sugar and vanilla until well combined.
Ice once cool and decorated as desired, enjoy!