Preheat oven to 300° and prepare baking sheets with a small amount of oil or baking spray.

Combine all the ingredients in a large bowl and work the butter into the dry stuff. (Use a mixer for a full batch, I find it easier to use my hands for a half batch.)

If dough is too crumbly to form a ball add a small amount of water (less than a 1/2 teaspoon) and continue to work it.

Roll the dough between 1/4" and 3/8" thickness and cut out several pieces with a cookie cutter.

Transfer to the baking sheet.

Re-roll the remaining dough and cut out more cookies until there's no more dough left. (I won't judge if you eat the remaining dough).

Place baking sheets in oven and bake until the shortbreads are light golden brown. Start checking at 25 minutes, but it can take up to 40 minutes depending on your oven and how thick the cookies are. Rotate the trays every 15 mnutes or so.

Remove cookies to a rack and allow to cool completely before decorating.

Measure 2 cups powdered sugar into a bowl (Depending on the size of the egg white, you may not use all of it.)

Separate an egg white into another bowl.

Sift about 2/3 of the powdered sugar on top of the egg white and use the whisk to blend them together.

At some point you'll need to switch to a wooden spoon and continue beating.

Add more sugar and continue beating until you have an icing that's very glossy and forms ribbons. (See the video because this is difficult to explain).

Add coloring if desired.

Apply the icing to the cookies using an icing bag fitted with a small tip in whatever manner pleases you.

Decorate with sprinkles or candy if desired.

Allow the cookies to sit undisturbed while the icing sets up.