In a bowl of a stand mixer fitted with the paddle attachment, cream the butter with the sugar until smooth, light in color and creamy. Add vanilla and egg and mix again until the egg is incorporated but don’t over whip.
Divide icing into small bowls and add food coloring. Add water as needed to get the royal icing to the proper consistency. To ice and flood cookies you'll need a 15-second consistency icing for outlining and details and an 8-10 second consistency for filling.
Add the icing to piping bags fitted with couplers and #2 round tips, or cut a small hole in the tip of the piping bag. Outline and flood the cookies first to create a solid background. Decorate with details after the first layer has mostly dried.