Cream butter and sugar in the bowl of an electric mixer on medium speed.
Add in the egg and vanilla.
In a separate bowl, combine the flour and salt, then stir into the butter mixture with the mixer on low.
Cover dough with plastic wrap and refrigerate for 30 minutes.
Preheat oven to 350 degrees.
Line a cookie sheet with a silpat mat or parchment paper.
On a floured surface, roll out the dough to a 1/4 inch thickness and cut into your desired shapes with cookie cutters.
Place the cookies on the prepared pan and bake for 8-10 minutes, until the edges are starting to turn golden, but not brown.
These cookies are supposed to not look done when you take them out, but they should be firm and set.
They will have a soft texture when cooled if they do not bake too long.
Transfer to a wire rack and cool while you prepare the icing.
To make the icing, sift the sugar into a bowl, add the milk and food coloring (if using) and stir.
If mixture is too thick, add a little more milk.
If mixture is too thin, add a little more sugar.
This is simple to adjust to get the right consistency.
Spread icing on cookies when they are completely cooled. The icing will set when they are completely dry.