Prepare the cookies. In a large bowl, beat butter and sugar until light and fluffy. Add vanilla and egg and beat until well combined. Scrape the sides of the bowl as needed. Add flour and salt and beat until just combined.
Dump the mixture onto a lightly floured surface and divide in half. Shape each half into disks and wrap with plastic wrap. Refrigerate for 30 minutes.
Preheat oven to 350°F. Line two baking sheets with parchment paper.
Working on a lightly floured surface, roll the dough to ½-inch thick. Cut the cookies using and ornament shaped cookie cutter. Place the cookies on the prepared pans and bake for 18-20 minutes or until golden brown. Remove from oven and allow to cool at room temperature.
While the cookies are cooling down, prepare the royal icing. Combine all ingredients together in a large mixing bowl. Beat on low speed until combined. Increase speed to medium high and beat for 8-10 minutes, adding 2-3 tablespoons more warm water, as necessary. Icing should be stiff enough to hold a peak when tested. Use this thick icing to outline cookies. Carefully add additional water to the royal icing to adjust to desired consistency.
Color royal icing with desired food coloring and decorate cookies.
If you do not have an ornament shaped cookie cutter, use a round cookie cutter and cut a small hole at the top using a straw.